Before the sequel, Sentinel 2: Earth Defense, Origin8 took us to another planet and had as mesmerized.

Sentinel: Mars Defense ($0.99)

In the future man decides that dominating one planet isn’t enough, so they take steps towards colonizing other worlds. The first stop is Mars, and everything goes well for a couple of years, until a garbled distress signal ends all communication with the mining colony that has been established there. As commander of the star ship Sentinel, you head out to Mars only to find out that it has been overrun by aliens. Now it’s your job to defend the colonies tower defense style. Your mission: don’t let the aliens declare “all your base are belong to us!”

Presentation

The first thing you’ll notice about the game is the music. In fact, I was a bit surprised when the opening thematic score was followed by simple scrolling text instead of some cool 3D intro movie. The music throughout the game is just as well done. The sound effects are also really good, each unit and monster having its own “voice”. Amazingly enough they don’t even sound like cookie cutter audio effects, either. Visually Sentinel is a feast as well.

The different creature types are well rendered, and they look good as they’re getting smoked by your weapons. The special effects for the weapons are nifty as well, especially the ion cannon. It’s pretty impressive to watch a group of turrets raining ammunition down on a horde of charging creatures. The thing that really surprises me, though, is that the game even looks half way decent when it’s fully zoomed out and really tiny.

Controls

The controls are pretty well done in Sentinel. At the bottom of the screen are all the units that you can build, and the available ones are lighted up. You click on the one you want to build and drag it until the circle around the unit is glowing green. You then let go to place the unit. To upgrade or sell a unit you click on it, and your options will appear on either side of the unit. You can also zoom in and out by pinching the screen, and as long as you aren’t fully zoomed out you can pan around the level by dragging the screen.

Game Play

I’ve never been a real big fan of the whole tower defense style game, and despite the strong production values and some decent twists in Sentinel, this game further cements my disinterest in the genre. The problem with tower defense games is that as a whole, they all play the same. You start out with an empty playing field, and pretty soon monsters will come knocking at your door. The creatures come at you in waves, and always follow the same path to their destination. To counter these villains you set up several structures that, if all goes well, will destroy the creatures before they get to you. In the end you will have successfully defended your main structure or you will have been overrun by the evil invaders. End of story.

In the case of Sentinel you have six weapons at your disposal. For of them are useful against both land and air creatures, including one that does ongoing damage. The other two are useful only against ground units, one of which will not harm the creatures but rather slow their progress. I was a bit surprised that there weren’t any weapons that were useful only against air units. Each unit can be upgraded three times, and if needed / desired, units can be sold back at any time. There’s a final unit called a drone that actually serves an interesting purpose. It can either harvest money for you, or it can repair your non-offensive units. There are two types of non-offensive units, which are the ones you need to protect. The first unit is a barrier, and the second is a base. Barriers act as a sort of “temporary” defense in case some creatures get past the units you build, but the guards atop the barrier are pretty weak compared to anything you can build. The base also has guards, but the same is true about them.

Besides the drone, which doesn’t generate much income, you also collect money by destroying monsters and by selling old units. Another thing I found interesting was that you can get interest on any money you haven’t spent when a round ends. This was a nice feature that I don’t think I’ve seen before in the tower defense genre (or in any resource collecting game, for that matter). You also earn a score based on kills. To augment your score, you get points for barriers that are in tact when a wave ends as well as whatever percentage of your base has been untouched when a wave ends. If your barriers and base are in perfect condition when a wave ends you get even more of a bonus.

There are a decent variety of creatures in look, though there’s not much difference in functionality other than speed and toughness. This is another reason that the formula Sentinel has adopted is starting to get old. If the creatures attacked the towers in addition to trying to reach their goal, then they could do some cool things with the creatures having different powers and such. Dungeon Defense is a good example of a game trying to head down that path. I did appreciate the fact that they at least have multiple starting points for the monsters in most of the maps. That was a nice change of pace. I was a bit surprised that there were only four maps though. That seems like a small fraction of what most tower defense games have.

Sentinel sports four different levels of difficulty, so it should accommodate most players’ aptitudes. Personally, my comfort level on most tower defense games is easy, so that’s where I stayed, but it goes all the way up to psycho for the ambitious. There are two battle modes: assault and endurance. Assault is a set number of waves against any one map, whereas endurance just tests you to see how long you can maintain a map against increasingly difficult monsters before you die. To me Assault felt like it was a campaign mode, so I was a bit disappointed when I had to go back to the level selection screen between each level. I almost expected some cut scenes to explain what was going on between each board.

Suggestions

Come up with some new game play elements. Part of the problem I’ve had historically with the tower defense genre is that most tower defense games feel almost exactly the same. There are actually a few examples on the iPhone of developers trying to break the mold, but I think in a genre like this developers need to be more conscious about trying to be different. In relation to the content that’s already there, more maps would be nice. Also, in Assault mode it would have been cool if the maps were somehow actually tied together, even if it was with some scrolling text or a simple one or two screen cartoon cut screen.

Conclusion

After the time I spent with Sentinel, I’m still not a big fan of the tower defense genre. However, Sentinel is a solid game that has some nice game play elements to it. I think a few more maps and some cohesion between the maps in Assault mode would go a long way towards making the Sentinel experience more appealing to those not as inclined towards tower defense games. Overall for me it was nice to try out but I don’t think there’s any staying power.