img_0166There's no question that Origin 8 had a hit on their hands with the original Sentinel: Mars Defense [App Store], a game that firmly held the title of my favorite tower defense game for a very long time.

Originally pitting players against a savage alien onslaught on the surface of Mars, Sentinel 2: Earth Defense [App Store] brings the fight home. A brief text introduction explains that despite your best efforts, the defense of Mars failed. Your drop ship, Sentinel, received a massive arms upgrade, and now you're forced to defend Earth. While not exactly a riveting plot, it gets the job done.

Where Sentinel 2 lacks in plot development, it more than makes up in overall gameplay. Now, before I get too far in to this, readers should be aware that I'm a tower defense nut. (Which comes as somewhat of a surprise to me since before I got in to iPhone gaming, tower defense games never interested me much.) Also, fellow tower defense nuts, I fully realize that labeling certain games in the genre as the "best" is as controversial as labeling the "best" Star Wars movie among Star Wars nerds. (Pre-remastering Return of the Jedi on Laser Disc of course.)


So, with that out of the way, prior to the release of Sentinel 2, I thought Sentinel was one of the best tower defense games on the App Store. The graphics were superb, the music was well done, the game performed great on my iPhone 3G (even while using the fast forward mode), but best of all, Origin 8 really nailed it with the balancing of the game.

The difficulty levels included made the game accessible for a wide spectrum of gamers, where "easy" was hard to fail and "psycho" left even tower defense experts scratching their heads looking for ways to refine their build outs to survive through the waves of beefed up creeps. Along with the standard array of direct damage, ranged, slowing, and area effect towers, Sentinel also featured an Ion tower which created a patch of red ion fire that caused sustained damage to anything that walked through it.

img_0170The most difficult decision to make however was when to deploy drones. At $150, drones were expensive at the beginning of each map, but slowly gathered additional resources to make them a very worthwhile investment over time. However, deploying drones too early could leave you cash-starved and unable to build the towers required to survive through the next few waves of creeps.

Each map also featured different temporary barriers that the creeps could destroy that served as bottlenecks to build your initial defenses. Each of these barriers generally had very few hit points, which when damaged invoked the secondary fuction of the drones, repairing.

All of these things together created a very refined gameplay experience that (especially in harder difficulty levels) made for a very technical game that really reminded me of all the time I had spent in the past doing things like tweaking Magic: The Gathering decks or trying to shave precious seconds off my Warcraft 3 or StarCraft builds. Or, on easier difficulties was just a fun, simple, and well done game.

With some extremely large shoes to fill, does Sentinel 2 live up to the original? In my opinion, a resounding yes. Sentinel 2 expands on the winning formula of Sentinel and provides some additional gameplay elements not found in the original.


The largest feature addition is that of your drop ship's newly upgraded weaponry. Each map has a destructible barrier that also serves as an energy generator. A tiny bit of energy is generated each round, which allows you to activate one of four devastating ship weapons when enough is stored. Homing missiles launches a barrage of missiles that target the creeps on screen, and when attack drones are deployed, three drones briefly appear on screen and go to town shooting things up with their lasers.

The two even stronger ship weapons are area effect attacks that are aimed by dragging the targeting reticule that appears on screen when they are activated. The time warp attack slows all enemies inside of the beam, and the orbital laser destroys anything it touches.

Along with your ship's upgraded weaponry is a second set of drones, which harvest energy at an increased rate. Now you can have three repair drones to gather resources to turn in to cash and repair your barriers, as well as three energy drones to power up your weaponry faster.

img_0171Also, in addition to the same set of towers from the original, Sentinel 2 features a Booster tower that increases the damage and range of all the nearby towers. The combination of all these new features add yet another layer of strategy on top of an already detailed tower defense game.

The best part? There doesn't really seem to be any "right" way to build your defenses most of the time. Going straight for towers that are fully upgraded with Booster towers deployed, focusing on repair drones, or prioritizing energy drones and using your ship weapons more often all have seemed to be perfectly viable defense strategies if each method is utilized properly.

Like the original, Sentinel 2 has four maps, each playable in four different difficulty levels. Once Assault mode is completed on a map, Endurance mode is unlocked which allows you to endlessly fortify your defense to see how many waves you can survive. However, something new in the sequel is the addition of Mission mode.

Ten missions, each increasing in difficulty challenge players to complete objectives such as surviving through waves of creeps by only using ship weaponry, build defenses with limited resources, as well as different scoring and time-based goals. I actually would recommend players try to make it through the missions before starting in on assault or endless modes because they serve as a great tutorial for some of the strategy required to do well at higher difficulty levels.

The music in Sentinel 2 is just as good as the original, with exclusive tracks written by Specimen A and this time around Origin 8 added full OpenFeint support allowing for online leaderboards, achievements, friends lists and chat shared amongst all other OpenFeint-enabled games.

If you're in to tower defense games, both Sentinel and Sentinel 2 are games you should already own. Each offer highly refined gameplay, music, and graphics as well as enough replay value to keep you busy for quite some time. (For reference, Sentinel came out in late February and I still play it every so often.)

If you happen to be new to the tower defense genre, Sentinel is a great place to start, especially with the current price of 99Β’. Since Sentinel 2 is an evolution of the original, if you enjoy the first, you will enjoy the sequel. Both are among my favorite games, and while there is no shortage of tower defense games for the iPhone, in my opinion, both Sentinel games are among the best the App Store has to offer.

App Store Link: Sentinel 2: Earth Defense, $2.99 - Sentinel: Mars Defense, 99Β’

TouchArcade Rating

  • TheXFrame

    I totally agree and great article. Sentinel 2: Earth's Defense is my favorite Tower Defense game out. I was shocked when I saw I had 4 Sentinel dropship weapons, new drones, booster towers, etc.

  • Carlos

    Agreed on Return of the Jedi. Sentinel 2 is also great

  • Stormchild

    Meh. Not a big fan of the original Sentinel. IMO it's one of the weaker tower defense games for the iPhone so far. It's very tedious and just not that much fun.

    GeoDefense is still the best path-based TD game. Very challenging.

  • Dark NRG

    I'm not a fan of the first either, and GeoDefense and Creeps are def the best, AND I agree with Return of the Jedi, AND cudos to mentioning Magic:TG, so I might just get this since Hodapp made it sound so good and threw me those bones πŸ˜€

  • ZZMitch

    I still hold a special place in my heart for good ole Fieldrunners! Maybe the only iPhone game to ever be mentioned in Time Magazine? :p

    I think my favorite TD right now is Star Defense. I believe it is much better then both Sentinel and Sentinel 2, with Plus+ further increasing my enjoyment with Star Defense. That is just my opinion though.

  • Ahiru

    sentinel reminds me of moon savage (ps3), but as for path-based td, i had more fun with 7 cities and currently with defender chronicles than i had with sentinel (haven't played that much so...)

  • Ahiru

    (er, actually i think it's savage moon, but whatever..._

  • Adams Immersive

    I like the missions idea, and the price is right! I always thought Sentinel didn't get the attention it deserved. (I bought it on looks, but the money-management is unique from any other TD--that's the hook that earned it a place in my TD collection.)

  • Fleabag323

    I like Sentinel 2, but for me, it's probably tied with GeoDefense, Star Defense, and SweetWater Defense, and surpassed by Defender Chronicles. My only complaint with mose TD games (except for SweetWater Defense) is that once you play with the fast forward, if you go back to normal speed, it feels like the game is in slow motion, then if you go back to fast forward, it doesn't feel like it's fast enough.

  • yacoub

    Great review.

    One cquestion is how you can (re)play a 4-map TD game for "quite some time" compared to other TD offerings in the App store. For example, I have Sentinel but I got much more replay value out of Besiegement and 7 Cities than I did Sentinel.

  • bfos7215

    Sentinel 1 was my favorite tower defense game. I know Star Defense is all of the rage right now. But, it looks like a fairly derivative game with a 3D that doesn't seem to change the game any more than if they gave you a flat map that you weren't allowed to zoom out of all of the way.

    Sentinel was also a standard TD game. But, it introduced enough new elements to make it stand out from the rest, for me.

  • Bobster

    Why didn't they do anything with the graphics?! It looks the same, and that's a bad thing to be honest.

  • Zak

    Another vote for how excellent Sentinel 2 is.

    I was disappointed in both Start Defense (brings nothing new but inability to see the entire map, which is a negative not a plus) as well as Defender Chronicles (I have to play the same level multiple times to power up enough to win it? You gotta be kidding me), but Sentinel 2 was a great surprise.

    I ended up deleting it from my iPhone, but only after I had beaten most of the levels on Hard and because I couldn't stop playing it (just 10 more rounds!), and it was wrecking my life. πŸ™‚

  • Ataru

    I'm done with tower defense games.



  • Tmac34

    I gotta agree with Ataru's comment above me. I personally refuse to, and will not allow myself to, buy more than 3 iPhone games of the same genre. I have 1 tower defense: 7 Cities, and I am satisfied with it. One thing I hate about the app store is when a dev is original and introduces an iPhone genre with his game, about a week later 10 more very similar games will come out, the devs being copy-cats. Whatever, just ranting I guess....

  • GGiant

    Tmac34.. Most devs and studios work independently (as they aren't going to tell the competition what they're up to) and it takes months to build a game. Usually they have no idea what the market is going to be like when they finally finish and get it approved. So a TD game you see now may have been in the works since last October, and if there's a rush of them it's not necessarily because the devs are all saying "me too!", but because they had a good idea for the genre back when there weren't so many games of that type out there, and it turned out a lot of others had that idea too.

    You don't have to buy what you don't want to of course, but look at it from their perspective too before you complain. Believe me, it's really depressing to be working on a game or app for all that time only to find out a bunch of other people had the same idea. Part of the problem here is the secrecy at Apple and the month-long turnaround time for approval too; you have no idea what else is in the works unless you actively try to talk to the competition.

    (There are plenty of shameless clones too, I have to admit, but I'm just talking about when a genre seems flooded.)

  • Ujn Hunter

    Wrong. The Empire Strikes Back is obviously the BEST Star Wars movie. πŸ˜‰

  • Adams Immersive

    TDs are a genre with huge room for creativity. Saying you only need one TD is like saying all platformers are the same! πŸ™‚

    And every TD I have on my phone (many!) meets my desire of having some unique twists and strategy elements. Some are FAR from anything else, others just take a fun genre and do something a little different.

    Sentinel indeed meets that standard. It's NOT the same as others, it has unique game mechanics. Is it for everyone? No; just like no one platformer is everyone's favorite.

    I wouldn't have thought TD could be so widely varied (or even fun at all) but now I know better!

  • dom

    I also have an obsession with these games. The creeps is still king for me. Just the right amount of depth, awesome graphics, and great gameplay. Sentinel is great though and im throughly enjoying sentinel 2. it is a touch easy though and I find the sniper is what always wins the game.

  • MTFox

    I never played a TD game before getting the iPhone, now have 11 on it of which I play 6 regularly. "Tower Madness" started it for me, so I still have a soft spot for it, but Sentinel 2 is the one for me at the moment. Love the new foes and weapons!

  • samsonsu

    I found fully upgraded booster brings most improvement to ion towers. Damage power of level 0 ion towers will be boosted from 30 to 510 (then 570, 630, 880).

    Boosters don't bring as much improvement to other towers. Only 15% or so in most cases.

    Strange setup.

  • cookiemonster

    how did i miss this td game?! looks bomb, checking it out now...

  • ArranX

    I have to agree with stormchild, I got the original Sentinel for free and wasn't impressed at all. In my opinion they're awfully designed games, enemies are just thrown at you down 1 or 2 routes over 4 levels. and "barriers have few hit points" is that a joke. You must be absolutley crap. And even if you disagree, surely it deserves criticism for being so tiny. Overall the series doesn't come close to games like DC in depth, variety or replayability.

  • ArranX

    Just seen Zaks comment. "Defender Chronicles (I have to play the same level multiple times to power up enough to win it?" Thats because your crap, dont blame it on the game. And dude, Sentinel has 4 levels. What a stupid comment.