Gaming Strategy Guide: Sentinel by Origin8

Despite the low price ($2), probably one of the best Tower Defense games for the iPhone is Sentinel by Origin8. The title is highly popular; no wonder a sequel has just been announced, slated for June. I highly recommend it - apart from the repetitive in-game music, it's excellent.

If you've ever played Tower Defense games, you already know how this one works. Therefore, I don't elaborate on the basics of the game (purchasing, placing, upgrading and selling weapons). Instead, I concentrate on real-world tips for surviving as long as possible.

(Maximizing) Score

The scores you get depend on mainly two factors:

- Base Bonus: this score depends on the state of your final, base, third gate. By default, you get 5000 points at the end of every wave if it's intact (there's no currently any damage done to it). If it needs repairing, you get far less (in general, under 2000) points.

- Barrier Bonus: this depends on how many of your additional gates are still intact. If both, you get (up to) 20000 points each wave, if only the second gate is intact, you can get (up to) 10000 points. If there's any (even the slightest) damage dealt to the gates, at least 5000 points are subtracted from this figure.

What does this mean?

1. While it's certainly easier to comparatively easily let your first gate be destroyed around wave 35-40 by, for example, not backing it up with Beams, the additional several thousand points each wave can become a game-changer in, say, whether you can get onto the worldwide highscore chart.

That is, if you stack up a lot of weapons to defend your first gate comparatively early in the gate (and, this way, you don't let your money stack up because of the high interest rates) instead of just relying on the (cheaper) weapons distributed along the long way to the second gate, you lose some money but gain a lot of additional points.

2. If the enemies deliver any damage to the gate early in the game (most importantly, the Arachnids are the most dangerous in this respect), try repairing them as soon as possible by purchasing the droids (or, at least one of them). This will result several times 5000 more points.


Laser (20 / 20; 1/2/4/10; the meaning of these numbers: original price / upgrade price [which is the same as the original price, except for Slow]; original strength / strength after the 1st / 2nd / 3rd upgrade): the cheapest weapon. At first (at least until wave 10) it alone is able to stop attacks without the enemies dealing much (if any) damage to your first gate. In mid-game (in Medium level, over 60-70 waves), it's not worth purchasing any more as the maximal damage it deals, when fully upgraded, is 10 only. However, before that (particularly before wave 40-45; after that, it's worth gradually switching to Beams), it's probably the best weapon against flying creatures (Winged Parasite).

Bomb (15/ 45 / 80 / 120): by far the most cost-effective weapon against weak ground enemies teamed up, particularly the very fast insects (Arachnids) and, to somewhat lesser degree, Mutants. Even usable in late game (over 120-150 waves in Medium level). I've found it far more cost-effective than even the two high-end weapons, Beams and Snipers. Make sure you keep at least 3-4 of them on your map even in late game.

Slow (100 / 50; 20/25/30/35): you'll want to use these weapons to greatly slow down your ground enemies. A must to use against the main bosses (coming every 30 wave), the Warriors, to slow down their movements and keep them in the range of your most powerful weapons.

Note: there is one weapon race that are not only immune to the slowdown effect of this weapon and, in addition, the Duo-Cranium Mutants. Therefore, you might want to do some micromanagement, particularly in late game where they have much bigger chance to get to the gate(s) and deal high damage to it if there's any Slow weapons on the map. That is, once you see there are Duo-Cranium Mutants coming, pause the game and quickly remove the Slow weapons from the map. As soon as another type of a ground enemy follows, just place them back. Given that these weapons are considerably cheap, you'll only lose 20% of their price ($20) when you sell one of them. This also means - as long as you use this kind of micromanagement - there isn't much point in upgrading them unless you have tons of money as, then, you'd lose more.

Of course, you won't necessarily want to use this micromanagement as it takes a lot of additional time. Or, only remove few Slows, not all of them. Decide accordingly: there may be cases when you don't at all need Slows to slow down Arachnids or Mutants because your weapons get rid of them without problems.

In the sample game below, I extensively use these kind of micromanagement.

Ion (150 / 150; 30 / 90 / 150 / 230): one of the best weapons, which can be used even in early game (from about wave 20). As with Bombs, it delivers splash damage. In addition, it makes every creature going through its slow-burning fire lose heath for quite a long time (about 4-5 seconds with turned on fast mode) with sustained damage. That is, it's worth putting more of them on the map separated so that the enemies are always affected.

Beam (250 / 250; 400 / 900 / 1400 / 2000): the most effective weapon. Interestingly, it's both faster and cheaper than the most advanced weapon, Sniper. Compared to the latter, it also has a slowdown effect (but not as long as that of the Slow weapons). The only disadvantage of them, compared to Snipers, is the reduced range.

Sniper (300 / 300; 300 / 700 / 1100 / 1800): it's possible you can pass further in the "survival" mode if you go for these weapons exclusively instead of Beams - at least this is what is the screenshot HERE, which shows the player to have got to wave 285.

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It's not known at which level (Easy? Medium? etc.) he did so, though.

Sample game

Let me present you a sample game, with all the screenshots and the explanation of what I've done. Note that I haven't presented a screenshot of several waves; the simple reason for this is that I haven't bought / sold / upgraded anything during them.

I started putting two Lasers on the both sides of Gate 1; of the initial $50, $10 remained.

In Wave4 (of Arachnids), I've purchased another Laser and updated it at the same time. I've done the same to the two other, existing Lasers. In addition, I've bought a Bomb and placed it in front of the gate, making sure its range still includes the gate.

In Wave5, I've further upgraded the leftmost weapon (on the immediate left of the gate)

I've fully upgraded the Laser in the center of the gate to deliver 10 damage.

In Wave 15, I've fully upgraded the two Lasers on the left/right of the gate to deliver 10 damage. I've also purchased another Laser (and fully upgraded it) and placed it in front of the gate.

I haven't purchased anything between Wave 15 and 29 so that the interest rate can grow. Right at the beginning of Wave 29, I've done the following:

- purchased the three droids. It was high time I've done so: while not much damage has been delivered to the gate so far, it being somewhat damaged (around Wave 4) has resulted in losing more than 25*5000 points. That is, if you do want to get as many points (but not money) as possible, don't wait this long with purchasing at least one of them.

- I've placed a Slow in front of the entrance - this is a must to slow down particularly the Warrior coming in the next wave

- I've purchased three Beams; their ranges all reach the gate. (With one of them, I've replaced the fully upgraded Laser behind the gate.) I haven't upgraded any of them; it's only after about 10-15 more waves that the strongest (non-Warrior) enemies like Winged Parasite can't be shot down with one (un-upgraded) Beam shot only; it's only then that Beams should be upgraded. And, the only Warrior coming in Wave 30, only needs some 7-8 shots at strength 400 (the default strength of Beams) to be killed. Paired with the Slow at the entrance which greatly slows down its motion, this guarantees it will hardly be able to even reach the gate before getting killed. NOTE: I could have waited until Wave 30 (the Warrior) with the purchase of these Beams (to let for more money flow in) as it's not Beams that are the most cost-effective against Arachnids coming in Wave 29 (but Bombs: see next bullet)

- I've purchased a Bomb, particularly against Arachnids (the ones just coming in this wave) and Mutants. I've put it in front of the gate.

I've purchased (and fully upgraded) two additional Bombs and put them at the entrance, with their range still reaching the gate itself.

I've purchased my first Ion, which I've fully upgraded at once. The splash damage of this, combined with the area damage of Bombs, is the best weapon against Arachnids (against which the Bombs themselves have become pretty weak and even better weapons are needed dealing area damage) and, to a lesser degree, Mutants.

In Wave 45, I've purchased another, fully upgraded Ion.

In the next wave, I quickly removed the Slow at the entrance so that I can deal maximum damage to Duo-Cranium Mutants - yes, I've waited this far with starting the (time-consuming) micromanagement as Duo-Cranium Mutants are comparatively weak and easy to shoot down.

... and replaced it in the next round...

...just to remove it again in the next wave as, again, Duo-Cranium Mutants come...

... while in the next wave Arachnids attacked; this is why I've replaced Slow.

In Wave 50, I've upgraded two of the three lasers to deliver 900 damage as Winged Parasites have lately become so tough that a single, un-upgraded Beam shot (of damage 400) no longer could destroy them. Around Wave 50, damage 900 is sufficient to kill them in one shot.

Neverheless, it wasn't sufficient: as the gate was already pretty much damaged, Winged Parasites got through.

Duo-Cranium Mutants again; accordingly, removed Slow at the entrance.

...only to replace it in the next round.

Seeing I need weapons to shoot down enemies reaching Gate 2, I've purchased a Beam and put in the left side of the second gate.

Again some micromanagement to make sure both Duo-Cranium Mutants and Arachnids are optimally attacked.

In Wave 60, I've added additional Slows along the path so that the existing weapons can kill the two Warriors without them reaching the second gate.

I've mass-added several Lasers in the upper left corner of the map because Winged Parasites have become too strong. (I've selected this place because, in the center, particularly around the main gate, I wanted to reserve place for more expensive Beams covering two paths at the same time.) In addition, I've added another Ion, fully upgraded, to inflict further damage to creeps.

Seeing the strength of the oncoming enemies attacking the gate and being afraid of their breaking through it, I've added a fully upgraded Beam behind Gate 2.

Wave 80: another onslaught on Gate 2; I ended up having to fully upgrade the Beam to the left to avoid the loss of the gate.

Wave 85: another (Winged Parasite) attack with severe damage done to Gate 2; I needed to build another , fully upgraded Beam to defend it.

Wave 86: micromanagement again: in order to be absolutely sure none of them gets to Gate 2 (which, as can be seen in the shot, was in a pretty bad state after the last Winged Parasite wave), I've removed most of the Slows (except for two) to deliver as much damage to Duo-Cranium Mutants as possible.

... only to replace them next turn because of the Arachnids.

Next, again a Duo-Cranium Mutants onslaught; I've removed Slows.

and the next wave was that of Arachnids, meaning I had to replace Slows.

Mutants have turned out to be far too tough in wave 96. Realizing this, it was too late to place several fully upgraded Bombs around gate 2 for defense: they, eventually, got through.

Winged Parasites again: I've added three, fully upgraded Beams in the center so that they not only protect the final gate, but also reach the path coming down on the left.

In Wave 118 (I haven't done anything for long; this is why I had a lot of money) of Duo-Cranium Mutants, I've only removed only one Slow (in the lower right corner) because I knew I could kill them off all without them reaching the final gate.

As can be seen, indeed little damage has been done to the gate. I've replaced the Slow in the lower right corner.

To be absolutely sure the three Warriors approaching don't destroy the main gate, I've added three fully upgraded Beams to the left of it (with the gate in their reach) and two on the right. I've also added another Beam in the upper left region, covering both incoming and outgoing path.

In the previous step, I've forgotten to add Slows in the right and left side of the main gate to slow down the oncoming Warriors. Of them, only one survived (at half health) the way there.

Arachnids having become too strong, I've deployed several (fully updated) Bombs in the top right corner, backed with an additional Slow and an Ion to maximize sustained area damage.

A series of micromanagement shots (removing / replacing Slows depending on Duo-Cranium Mutants) with no real purchases except for a bunch of (still non-upgraded) Beams in Wave 141.

Duo-Cranium Mutants have become so tough that I had to both purchase three new (fully upgraded) Beams to the right of the main gate and also fully upgrade all the beams purchased in Wave 141.

I've replaced the Slows.

... and used almost all my money (leaving some 2300) to purchase a lot of fully upgraded Beams to kill the attacking Warriors.

Duo-Cranium Mutants again and some micromanagement: selling all Slows not to run into problems.

I've postponed my next purchases until the next Warrior onslaughts (Wave 180). Then, I used all my money (I had well over 20000) on fully upgraded Beams. I've replaced all Lasers and some Bombs with them.

Some micromanagement again: deploying and selling Slows depending on Duo-Cranium Mutant attacks.

For some waves, I've haven't replaced Slows to speedup play and reduce the need for micromanagement. As can be seen, Arachnids have just managed to get to the gate but they haven't been able to deliver much damage to it.

Before, however, in the next Arachnid wave, just to be on the safe side, I've re-deployed them.

And again removed them because of the Duo-Cranium Mutants

Before the Warriors attacking in wave 210, I've spent all my money (about 15000) on fully upgraded Beam and, in the corners, Sniper towns. There isn't anything else on the map, except for some Ions and one Bomb - and, of course, Slows.

However, not even these could stop the Warriors: three of them have reached the gate at half health. You can easily guess the outcome...

Finally, a high scores list showing these results, along with some older ones.

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<p>Werner Ruotsalainen is an iOS and Java programming lecturer who is well-versed in programming, hacking, operating systems, and programming languages. Werner tries to generate unique articles on subjects not widely discussed. Some of his articles are highly technical and are intended for other programmers and coders.</p>
<p>Werner also is interested in photography and videography. He is a frequent contributor to not only mobile and computing publications, but also photo and video forums. He loves swimming, skiing, going to the gym, and using his iPads. English is one of several languages he speaks.</p>