Every now and again, a game comes along that redefines a genre. Sentinel 3: Homeworld is just such a game. It is a tower defence strategy game that pits you against the usual waves of ever more formidable enemies, but it does so with such flare and panache that we felt it worth a mention here. Firstly, you have a Commander, a giant robot (Mech Warrior style) that forms the basis of your defence. This commander "levels up" as you progress through the game and attributes such as "shot strength", "health", "PSI (the effect he has on nearby turrets)" and "Strength (his hand to hand melee damage)" all increase gradually as you level up.
With new levels come new abilities that can be purchased (with in game credits, not real cash). The ability to teleport or to auto repair nearby barriers, or to upgrade to "triple shot" to target 3 enemies similtaneously, or to rain plasma bombs on nearby enemies are all attributes that are essential if you want to succeed at the higher difficulty levels.
The turrets themsleves are many and varied and you have to select your weapons group before entering each level. This makes for another strategic challenge as different maps lend themselves to different weapons. Also, a lot of the more powerful weapons are locked until you progress. You also have to buy weapons slots - so initially you might be able to select an arsenal of 5 weapons, but with experience you can purchase more slots so as to face the enemy with up to 8 different weapons at your disposal.
Weapons can be upgraded during the game and also boosted by a booster turret, which effects all turrets within one space. Wise use of a booster can make all the difference.
In addition to the normal ground weapons (or turrets) there are also "Sentinel Weapons", these are super powerful weapons that can be lauched from your orbiting spaceship. Whilst they are powerful, they use up your energy quota and it is essential to time your use of these wisely if you want to win. There is no point in wasting the "Orbital Laser" to wipe out a bunch of lesser enemies when there is a monster boss just about to make an appearance!
Grouping turrets is another essential element, using the 'Slow' (a different turret for ground and air enemies) in conjunction with a cluster bomb or artillery (for ground enemies) is a great way to cut through swathes of the beasties before they get to your barriers.
Two different types of DRONE are also available to spend your hard earned credits on, one handling Barrier Repair (they also harvest credits when barriers are perfect) and the other being Energy Harvesting drones (to power sentinel weapons). You can purchase up to 3 each of both types - but the credits used to buy them mean you have less to buy or upgrade turrets, so, again, strategic thinking is required.
The 4 difficulty levels (Easy, Normal, Hard and Psycho) are nicely spaced. I completed the first campaign (20 levels) on Normal and found it pretty easy going. Then I made an in-game purchase to buy the 2nd campaign (an extra 15 levels) and set the difficulty to hard...a very different story! This takes real thought and planning - so far I have got to the 14th of the 15 levels - but it has been a real hard slog!
The Psycho level (which I intend to attempt the first campaign with next!) is a beast :-) But it is good to move away from the campaign to the "endless" mode set at psycho level to gain experience. Experience and level ups earned in endless mode carry across to the campaign, so you can train up your commander in endless mode to gain the requisite skills to face off the harder campaign levels.
I can't recommend this game enough to tower defence nuts, it is a cut above other IOS Tower Defence games such as Star Defense etc and well worth the money! Get it!!