After defending Mars from a vicious alien assault, those pesky space invaders have set their sights upon a new target: Earth. Let's not worry too much about the back story for this fun little App though - there are too many aliens to kill!

Sentinel 2: Earth Defense is a tower defence game where tons of Zerg and Protoss wannabes make a run on your decidedly Terran bases. Each level has at least one point of entry for the little Martian toolbags, and each level has at least one barricade that they must break through before they can reach the final gate. There are seven different types of towers in Sentinel 2, ranging from the small turret with constant fire and a very small line of sight to a sniper with huge damage and almost infinite range. In many levels, the key to success is a balance of different towers that perform different functions, although certain levels might be better suited for fully upgraded lower end turrets. It's guess and check for the most part, and it's always fun.

Each barricade does something different however, so in theory different levels require different strategies. However, when it really comes down to it, the strategy is always "kill the aliens before they rip apart your women and children" - and at the end of each wave you'll receive different bonuses for whichever kinds of barricades are still intact or unharmed. If this sounds confusing then that's because it is, at least at the beginning. This is mostly because the in-game tutorial on the first level of the first campaign puts so much information in front of you that you'll have to go back multiple times to register it all.

If you've ever spent time with tower defense games before though, it should be just familiar enough territory to jump right in with relatively few hang-ups, even if there's still a lot to learn with Sentinel 2. First of all - and most importantly - is the game's namesake and your godsend: the Sentinel. Apparently there is a large ship overhead that can shoot at your enemies with enormous guns, supplementing your structures and saving the day in a time of dire need. It's basically a completely new concept to the game and very little amount of time is spent explaining the logistics of it, so it takes a while to realise that with each wave your Sentinel is charging itself.

You can purchase two types of drones too (with a limit of three of each) that fly around the screen as well. One group of sentinels can collect money and repair barricades that those mean-spirited Martians have attacked, while the other group helps you charge your Sentinel more quickly, which aids you in killing large groups of aliens quickly. It's a tricky procedure due to the fact that the Sentinel has four different types of weapons that it can shoot, and each one requires different charging and has different effects. Basically, what it boils down to is that the longer your Sentinel charges, the more lethal is weapons are. Also unique to Sentinel 2 are the booster towers, expensive structures that affect the surrounding towers without actually doing any damage themselves. For instance, weak gun towers may receive a +60 attack while simultaneously broadening their range of attack. Placement and employment of booster towers is critical and you won't get very far without utilising these to the utmost.

There are only four different levels in Sentinel 2, but luckily the level designs are hands down the best I've come across in any tower defence game. Certain maps restrict the types of structure on offer, which is typical for the genre and makes for challenging simple maps and incredibly complex later levels. Because of the different kinds of barricades (there are three), Sentinel 2 has a very unique set of maps, where strategic defence of certain barricades is the key to survival. For instance, in the level "Breakthrough", one barricade blocks two paths while the other blocks only one, forcing you to balance your structure placement accordingly.

There are two modes of play on offer in this App - Campaign and Missions. The campaign allows you to play through each level across four difficulties, from the extremely simple Easy to the insanely difficult Psycho. After beating the campaign you can take on the ten missions, each of which have different objectives and resources available; these are really tough and definitely worth the time for those who think they've mastered the game.

For an experience as complex and deep as Sentinel, there really aren't enough different enemies available. Each group of the six different kinds of aliens that try to infiltrate your base have unique strengths and weaknesses, but in the end they become slightly repetitive. More enemies would have been nice, but the ability for more than one type of bad guy to appear in each wave would have been even better. Sadly though, each wave only inflicts one type of Martian upon you, and this can eventually look sort of dull. Perhaps the same enemy with a different colour would have broken it up just enough, but that is nitpicking.

This minor complaint aside, at all times the graphics are technically impressive but usually too small to really see the details. In order to see that each individual building is actually quite well rendered, you need to zoom the screen in enough for it to be very tricky to carry on playing the game. Perhaps, for this reason, tower defence games on those relatively small Apple screens work a little better with simpler graphics with brighter colours. The music meanwhile is fine, if nothing to write home about; however, with an option to play music from your iPhone or mute it completely, there's really nothing to complain about - nothing beats wiping out hordes of aliens while listening to some clever mix featuring the Halo theme!

Hands down the worst part of Sentinel 2 is the inability to quickly restart missions; the navigation of several menus with several selections is required before finally restarting a mission. This can be a real buzzkill in later missions where you only get one chance to set your structures up correctly. There was a certain mission that I restarted something close to fifty times; I wasn't upset that the game was so challenging, but rather that I had no easy way to keep retrying

Sentinel 2: Earth Defense is a highly enjoyable and very challenging tower defence game. It's tough to judge Apps on a typical videogame scale but quite a bit easier to say if it's worth the money or not; with decent presentation and addictive and engaging gameplay, if you're even slightly partial to tower defence games (or puzzle games in general) then for the meagre price of £2.99, Sentinel 2 belongs on your iPhone or iPod Touch.

Reviewed by Nic Vargus for AceGamez (All Rights Reserved).