Tower defence games have been extremely popular since the yearning for casual gaming began, and the Internet is so awash with them now that it's hard for individual titles to stand out. Generally, this is because it's a game genre that can lack originality if not done correctly. Today's App of the Day, however, does not lack in this area.

Forget birds versus pigs, it's the eternal struggle between monkeys and, er, balloons that's got Pocket-lint hooked...

Bloons TD 4 HD

iPad / iPad 2 (also available on iPhone / iPod touch)
£2.99 (on offer)

Scottish games development company Digital Goldfish has been documenting the battle between monkey and balloon for several years now, albeit through browser-based Flash games.

First came Bloons, a puzzle game that had you play as an ape who had to throw darts at a range of differently-coloured balloons, some with super-powered properties, and the art was in clearing the screen of a set amount before progressing to the next level.

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Then came Bloons TD (or Bloons Tower Defense, as it was called before realisation that the latter two words are trademarked - no, really), and an enormous cult following amongst casual gamers emerged.

Using the same balloon properties, the game switched the action to an overhead view, and allowed the placing of monkeys with different powers along the edge of the "bloons" pathways. Armed with darts, spikes, flames and the like, a level was only complete when a wave of the air-filled agitators was destroyed. Then a harder wave would appear, and so on. Basically, much like any other tower defence game, sorry, TD game.

However, the fun in Bloons TD was always in the wackiness of its characters and features, and the latest instalment Bloons TD 4 HD is the pinnacle of the franchise.

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It's also the first to arrive on the iPad (and iPad 2). All the Bloons games, TD and otherwise, have appeared online (and are still available to play for free on Ninja Kiwi), while only the iPhone has been privvy to Bloons and Bloons TD in the past. We have had Bloons Super Monkey, a shoot-em-up featuring one of the characters, on the tablet device before, but never the crème de la crème of Digital Goldfish's line-up.

Boy was it worth waiting for though.

Bloons TD 4 HD was born to be played on the iPad. Its controls are dream-like, with the different monkey types lined up on the right-hand side, and simple drag and drop bringing them into play (the iPhone version requires a separate tower selection menu to be accessed in order to avoid obscuring the action - it can get fiddly though). And the colourful, simple graphics pop out of the screen (using balloon-related terminology).

There's plenty of stages to complete, and a stack of rewards. For starters, you only get a certain number of available monkey types accessible at the beginning, and they unlock as you pop more and more balloons. Of course, they'll still cost progressively more money to buy, and you'll have to balance your in-game funds to afford them, but once you've earned them once, they are available from the off of subsequent plays.

Further upgrades can also be earned by completing stages on the hardest level. It's a move than rewards the persistent gamer.

And persistent you will be, as there are few games more addictive than Bloons TD 4 HD on the iPad.

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Our one gripe is that, on the original iPad, when you've got a lot happening on screen (towards the latter end of the more complex levels), the graphics stutter and the frame rate drops considerably. We would imagine that this would be better on the iPad 2, but didn't have one to hand to test it on (yet). If we do soon, we'll update you.

Either way, it doesn't really detract from what is, along with Angry Birds and Cut the Rope, an essential game for any iPad owner, first or second generation.

Are you a big Bloons TD fan? Have you played the iPad or iPhone versions? Let us know your thoughts in the comments below...