Tower defense games are a favorite among mobile gamers, but many of them don't offer much in the way of variety or replay value. Bloons TD 5 looks like a child's tower defense game, but it's surprisingly deep, entertaining, and rewarding. Plus, Impoppable mode will challenge even the most veteran of the hardcore tower-defense crowd.

What's a Bloon?

Bloons TD 5 is one of a long line of games called Bloons; the first games were simple physics challenges in which you had to get a monkey to throw a dart that would pop a number of on-screen balloons.

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Soon after these games were released, however, Ninja Kiwi released a number of spin-offs, including Bloons Pop 3, Super Monkey, Hot Air Bloon, and Bloons Tower Defense. There have been six Bloons TD games — the five regular ones and a competitive one called Bloons TD Battles. We'll be taking a look at the fifth in the standard series here: Bloons TD 5.

How Does It Stack Up?

Despite the innocent theme, Bloons TD 5 actually packs quite a punch for a mobile tower defense game. There are over 40 tracks; a co-op track; 21 towers, each with 8 potential upgrades along two paths; 10 special agents; 10 special missions; 250 random missions; and four difficulty modes. Ninja Kiwi plans on releasing more updates in the future, making this a phenomenal deal for a $2.99 game.

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There are a number of different types of balloons that you'll see during the game — from the simple, slow red balloons to the faster yellows and pinks, blacks and whites, zebras and rainbows (yes, seriously), lead balloons (no joke!), and finally ceramics, which take ten hits to break open. After the ceramics come even bigger enemies — the MOAB (Massive Ornary Air Blimp), the BFB (Brutal Floating Behemoth), and the ZOMG (Zeppelin of Mighty Gargantuaness), the biggest, baddest bloon of them all.

To deal with these sinister inflatable enemies, you get to use towers that include a dart-throwing monkey, a machine that shoots tacks, a sniper monkey, a ninja monkey, a boomerang-throwing monkey, a monkey piloting an airplane, a monkey pirate ship, and a number of others.

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By unlocking upgrades to each tower as you gain experience, you'll be able to upgrade your monkeys to throw spiked balls, launch multiple shurikens at once, summon lightning bolts, and shoot lasers. My personal favorite is the Aircraft Carrier , which allows your monkey-piloted ship to launch a number of tiny airplanes that fire darts.

In addition to upgrading your towers, you have the opportunity to build specialist buildings that give your towers special upgrades; you can also purchase general boosts that give your towers more popping power, increased attack speed, and other bonuses. If you're going to progress through the more difficult tracks, you're going to need them!

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To upgrade towers and get some general bonuses for your monkeys, you'll need to spend the in-game currencies: tokens and monkey money. Both can be used to buy various types of upgrades, and $3,000 in monkey money will unlock Fast Track mode, where you start each map at level 26, helping you get to the higher levels more quickly.

You earn both tokens and monkey money by completing levels, and you get some monkey money each time you fire up the game for the first time in a day. You can also purchase tokens and monkey money from the in-game store. You can buy in amounts from $0.99 for a few monkey tokens or dollars all the way up to $49.99 for a mountain of monkey money. So far I haven't spent any money on the currency, and I'm progressing at a pretty good rate.

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To make it to the highest levels, you'll be building a vast array of highly upgraded towers that will have to pop tens of thousands of balloons each round. Depending on your device, this can create some significant lag. As you can see in the screenshot above, things can get extremely packed. On my Samsung Galaxy S3, I don't see any lag until round 80 or so, and nothing significant until closer to 95, so it hasn't been much of an issue.

The Latest Addition: Impoppable Mode

In its latest update, Bloons TD 5 received a new difficulty level. Now, in addition to easy, normal, and hard, there's Impoppable, which presents a nearly insurmountable challenge. In easy mode, you get 200 lives, the towers are cheap, and the bloons are pretty slow. You'll win an easy track by getting to level 50. In hard, you only get 100 lives, the towers are more expensive, the bloons are faster, and you have to get to level 85 to win.

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So what's Impoppable like? It's impressively hard. You start with a single life, and the towers are more expensive than ever.

It's hard. Really hard.

Unfortunately, you can't just jump right into Impoppable mode. You'll have to work your way up to level 35 first by gaining experience in the easier modes. It shouldn't take too long to do this — a few days of consistent play should do it. You probably wouldn't want to take on Impoppable without first unlocking all of the towers and upgrades first, anyway.

If you've blazed through Robo Defense or Kingdom Rush, give Impoppable mode a try. You'll almost certainly find it a welcome challenge!

The Verdict

Bloons TD 5 is one of the most entertaining, most rewarding mobile tower defense games. Getting to level 100 on any given track might take you an hour, so there's a lot of gameplay value for a $3 app. The humorous tone of the game, family-friendly theme, and an impressively huge amount of variety make this one of the best tower defense games for Android. You'll be spending hours of potentially productive time on this one. (Don't say I didn't warn you.) And the new Impoppable mode provides a challenge that will test even the most experienced tower defenders.

If you're a fan of tower defense games on any platform, and you don't mind placing small towers on a small screen, Bloons TD 5 is a great purchase. Who doesn't like the idea of a monkey destroying thousands of balloons with a gatling gun that shoots darts?

Download: Bloons TD 5 for Android, iOS ($2.99)

Have you played Bloons TD 5? What do you think of impoppable mode? Share your thoughts below!