By Werner Ruotsalainen on Tue, 06/30/2009
Probably the best Tower Defense game out there is Sentinel by Origin8, which is offered for $1 only and is an excellent buy. (See my Strategy Guide HERE.)
The sequel to Sentinel is considerably better than the first version. Most importantly, it has four always-available, additional weapons freely available (depending on your "energy" level, which depends on how many still not destroyed gates you have and whether you’ve bought new "Energy" droids):
There is a new tower, Booster Turret, (greatly) helping to extend the range of any nearby towers, particularly when maxed out.
There is a brand new, flying enemy, Arthropod Parasite, which is a tough guy: against it, you can mostly only use high-damage weapons like beams and snipers – or the Orbital Laser:
There is also a welcome addition to the game: upgrading a tower to the max offers a big leap in usefulness. For example, the otherwise slow Sniper becomes two times faster; Lasers and Bombs will be able to attack three enemies at the same time etc. The following screenshots show some of these upgrades:
There is a nice, highly recommended mini-review HERE, in the dedicated Touch Arcade thread. (Note that it states "New ones have been introduced that can teleport from one location", which isn’t entirely true – Two-Headed Mutants already existed in the first part.)
A mini Strategy Guide
Let me show you how I got to round 100 on the first map in Easy mode. Note that I’ve excessively used the Orbital Laser to take out Arthropod Parasites and Warriors – the two worst kinds of enemies. Arthropod Parasites, in general, come in the 15th and the 27th round (after a Warrior). This, unfortunately, means if you use Orbital Lasers to take out them in the 27th round, you won’t be able to use this weapon against Warriors in the 30th one – the energy just won’t fill up in this short time to enable Orbital Laser again. (Of course, in every 15 + n*30 rounds, you can safely use Orbital Laser against Arthropod Parasites – it’ll have plenty of time recharging.) That is, you’ll be, sooner or later, overrun by either of the Arthropod Parasites or Warriors.
I’ve started the usual way: a Bomb behind the first gate. (In the following screenshot, it has already been upgraded to Level 3). After that, I waited a bit and slowly (in order to enable my money to gather) upgraded my weapons. I always used Orbital Lasers to take out Arthropod Parasites; to take out Warriors at this early stage (the first three waves at round 30, 60 and 90), some Beams were sufficient. (In turn 30, for example, a simple Beam at level 1, next to a booster, next to the gate, was sufficient.)
Even at round 98 I had no problems fending off the enemy waves with a pretty minimalistic approach:
(Note the almost full lack of energy – that’s because I used Orbital Laser to quickly kill Arthropod Parasites in the previous round.)
Around round 117, I’ve deployed five, fully upgraded new laser towers (with a fully upgraded range booster in between) and, in the entrance, several Bombs. (Note that Bombs aren’t as needed as in the previous Sentinel as, now, fully maximized Beams attack three opponents at a time, which means they’re way more efficient against Arachnids. This is why, later on, I’ve sold all Bombs and gone the fully upgraded Beam route, particularly because their additional anti-air capabilities.) As you can see, not even these towers were able to stop Arthropod Parasites; this is why I had to, finally, turn to Orbital Lasers. (Warriors at round 120 could easily be killed with these towers, without using any of the new, additional weapons.)
After this, I haven’t purchased anything until round 143, when I’ve fully upgraded my Bombs in the entrance:
During round 147, seeing the currently available (eight) beams won’t suffice, I’ve quickly deployed another eight Beam towers to kill Arthropod Parasites:
After this, I haven’t done much until round 165, when I put some new Beams next to the entrance:
It was only in the 211th round that I’ve continued purchasing new Beams. It’ s there that I’ve sold all my Bombs and Lasers to free up space for the new Beams:
As you can see, I had no energy at all because I used Orbital Laser in the previous round, agains Warriors.
After some rounds, I’ve deployed some towers on the ridge on the lower left:
Finally, in round 225, I’ve finished deploying Beam towers (along with Boost towes in between), all maximized.
The following screenshot shows the remaining Arthropod Parasites, after killing most of them with Orbital Laser:
As you can see, the remaining few already had little health while still far away from my first gate.
In round 267, I’ve killed most of the Arthropod Parasites with Orbital Laser (hence the low energy level); the ones remaining have just reached the first gate (but haven’t managed to break through it):
In round 270 (Warriors attacking), having used the weapon in round 267, I still had no sufficient energy to power Orbital Lasers (not even slowing down; the first two weapons are almost useless against powerful opponents like Warriors); therefore, I could only rely on the Beams already present on the map. As you can see, some of the Warriors still had a great deal of health when approaching the gate:
And yes, they did reach the gate:
…and break it:
In round 297, I’ve pretty quickly realized Arthropod Parasites just can’t be killed without using Orbital Laser. Just take a look at the following screenshot: while the Arthropod Parasites in the front have already walked for quite a long time, most of them are almost entirely healthy. That is, I couldn’t refrain from using Orbital Laser to stop them; I was pretty sure they would, otherwise, have broken through the second gate.
The rest (few) Arthropod Parasites were, then, easily killed by Beams:
In round 300, I couldn’t kill all Warriors and, as you can see, my energy was still insufficient to use Orbital Laser. Finally, they have broken through:
There’s another mode – missions. They are pretty hard – you will definitely want to practice in Campaign mode first.
The first mission is easy to do even if you’re a newcomer to Sentinel 2 (but already know the towers from Sentinel 1). The second is much-much harder – I needed to re-try at least 30 times to get through it. To be able to successfully finish it, you’ll need to wait until both first gates are destroyed. Do NOT try to attack the Parasites when they’re in front of the two gates – it won’t work as, then, some of them will still not be there. Just wait until the first of them reaches the upper, horizontal path and start moving left, already in one line. Then activate Orbital Laser and quickly move over them all. This, in some tries, will result in killing them.
If at least one of them remain, don’t wait until it reaches the final gate – you’ll be only able to purchase one Beam and won’t be able to max it. Without maxing, you won’t have any chance of even one parasite.