Free!  That awesome word.  Origin8 is giving away Sentinel: Mars Defense for one week only.  This game was so highly rated that it got a sequel.  This sweet deal should start at 6:00 PM GMT, but is not yet active on the App Store as of publication.  So get that iPhone ready for a great tower defense game.

Developer: Origin8
Genre: Tower Defense
Price: $2.99
Verdict: Extreme fun with hours of replay value.
Pro: The ability to pause the game and speed up the game is a great feature
Cons: Need to be able to remove new player notifications

Following the critically acclaimed Sentinel: Mars Defense, Origin8 has created another masterpiece with Sentinel 2: Earth Defense. The game takes place two months after the failure on Mars. Reinforcements never arrived and all personnel were lost. In preparation for the coming attack, your dropship, Sentinel, has been upgraded with an arsenal of devastating weaponry.

On a platform where tower defense has flourished, Sentinel 2 has taken some progressive steps in the evolution of the tower defense (TD) genre. Some of the new features within this game include the ability to build mobile attack drones, construct boosters that will increase fire rate, boost power to all surrounding offensive units, and it includes the upgraded sentinel weapon system I will call “Special Weapons”. As you can see from the below screen capture, there are four different Special Weapons from which to choose. As one progresses through waves of attacks, harvesting drones will be gaining vital resources to fuel these weapons. The more your drones can gather the faster you will be able to launch one of the four Special Weapons.

The monetary arrangement that is implemented in Sentinel 2 is similar to the vast array of other TD games. As the player destroys the aliens through waves of attacks, money is acquired based on the level of the attacking alien wave. With these funds one can construct a variety of weapons, i.e., turrets, grenade launchers, slowing pulse towers, electrical towers, and boosters. The player can also purchase repair drones that repair the gates if it takes damage from the attacks.

I have played many TD games before, and the format is much the same. You setup towers, wait for the next wave, and then build new towers or upgrade existing ones. However, Sentinel 2 provides a little more challenge than one might expect. For example, one of the alien waves when hit by the slowing pulse tower is teleported away from the attack. This allows it to bypass any of the defenses the player may have in place assuming the player has not built up defenses further down the line.

Sentinel 2 has definitely given me the most exhilerating experience I have ever had in a TD game. When lying in bed with closed eyes, you can feel and hear the humming of the booster generators going off and turrets firing at a break neck pace. You sense the smell of gunpowder and dust flying through the air as a grenade explodes in the trenches.  With aliens charging at you, as a last defense, you use a singular shock tower blasting beams of electricity over your head. Your hair is now standing on end.  The alien explodes and there you are alone but ready for the next wave.

The previous campaign on Mars was a failure. The colonies fell to ruthless creatures. Are you going to let them take over earth without a fight? Prepare your sentinel and dig in for the long haul, because you aren’t going anywhere soon.

Developer/Publisher: Origin8 Technologies
Price: $0.99
Verdict: Brilliant fun, perfectly priced and well worth a purchase.
Pros: Simple but oh so addictive.
Cons: Could maybe benefit from a few more maps/levels.

Sentinel is that rare breed of iPhone game, at least for me. It’s one that attempts to have a story! Well I say a story, it has a small sequence at the start that tells a brief tale about a human colony on Mars being invaded by aliens. It’s all told through the exciting medium of text. Yeah ok, maybe not so exciting after all. But Sentinel is fun, honest!

A tower defense game, Sentinel is all about protecting your base from an almost constant stream of alien invaders. The alien insurgents come in waves, each level has a certain number of waves before you’ve succeeded at winning the level. To keep your base from being destroyed by the alien scum, you have to build gun turrets to defeat the aliens. There are up to 6 different types of turrets that you can use. One just shoots bullets at the aliens, one fires grenades within a certain range, one slows the enemy progress down, one fires red bullets from a longer distance and the final two most expensive turrets do similar things but from a much wider range and much more effectively. Each of these turrets can then be upgraded up to three times, to become even more powerful.

The levels are quite small, only the size of the iPhone/iPod touch screen but that’s all that’s needed for this simple, but addictive game. It was the sort of game that encouraged me to have ‘just one more go’. Essentially all that I could achieve was a higher score but it was still strangely addictive. There are two different modes of play: Assault and Endurance — although both are essentially the same. In Assault Mode, you can reach a conclusion, while Endurance goes on for as long as you can possibly hold out. Sentinel integrates well with the iPhone so that it automatically saves if you have an incoming call or SMS message so that your progress isn’t lost. I found  that each Assault mode level took me around 30 minutes to complete, so it was handy that I could drop out of it at times without missing out on anything.

When you take into consideration that Sentinel is a mere $0.99 you really can’t go wrong with it. In this sort of price range, I feel myself slightly limited to many, many puzzle game clones so it was refreshing to see something a little bit different. As you might be able to tell, I really did enjoy Sentinel. It’s simple but that’s no bad thing at all. It’s excellent fun, just think of it as a starter rather than a main dish in the world of video game cuisine.