4Towers: Onslaught [$1.99] is the latest tower defense game from LambdaMu, the makers of Infinight [$1.99/Lite] and Dice Soccer [Free]. If the name of this game sounds familiar, that’s probably because it was originally released mid-2010 as 4Towers Combo TD: Classic Edition [99c] .

Onslaught is a reboot of their Classic game, as it delivers additional levels, bosses, an features, plus improved vector-style graphics and sounds. The first 22 of the 28 levels are taken directly from the Classic game, but look better and include the addition of equipment which grant special abilities.

4Towers: Onslaught features 13 different varieties of enemy (or “creep"), each with different health, speed and abilities. Creeps travel in waves, along a set path. You set-up towers to destroy them, before they reach the end of the path. Anyone who plays tower defense games will be completely familiar with this concept. The creeps have abilities such as turning invisible, spawning swarms and healing other creeps. And each level features one unique boss to defeat, so that’s 28 bosses.

This game is quite similar to GeoDefense [$1.99/Lite/Review], which is an excellent tower defense game from early 2009, although there’s three main differences. Firstly, there are set positions in Onslaught where you can install a tower, rather than being able to position towers anywhere. Secondly, some of those positions are specifically allocated for combining two towers together, as a combo-tower.  And thirdly, instead of failing once 10 creeps escape, the remaining health of each escaping creep is removed from your own health, causing you to die if your health reaches zero.

This game is named “4Towers" for a reason. The four basic towers are red (blast tower), blue (slow tower), green (weaken tower) and purple (cannon tower). By combining two towers together, you can create 18 stronger combo-towers, with new abilities, to fight off the swarm.

Here’s a couple of examples:  Two linked red towers form a rapid fire tower, blue and green towers form an immobilizing stun tower, while red and green towers form a sniper tower which specifically targets creeps with the highest health points (HP). Four of the combo-towers are achieved by placing towers on special charge platforms, for extra powerful attacks once charged up, although these have a long cool-down period.

The gameplay controls are simple and intuitive. Towers are built by tapping an available marked position, then selecting the tower type. You can tap an existing tower to upgrade or recycle it. Recycling means you get an instant refund for all money spent on that tower so far, enabling you to create a new tower elsewhere. This redeem-and-relocate technique can be used strategically mid-game, to move towers back in front of a wave of creeps after they’ve passed by.

There’s a “fast-forward" button, to make the action play out faster, or you can press pause to make your moves without pressure. A radar shows the next approaching wave of creeps, and tapping it causes the next wave of creeps to spawn immediately. This allows expert players to confront two waves of creeps simultaneously for higher scores. You can also unlock Veteran and Elite modes for more difficult and prolonged game-play.

When you complete a level, you’re rewarded with biomatter (in-game currency) and the next level is unlocked, even if you didn’t manage to defeat the boss. However, it’s only by killing the boss that you collect the items it drops.  Collecting equipment will give you abilities or tower modifications, boosting your stats. You can unlock items using biomatter or if you’re impatient, via in-app purchases. Game Center rankings and a healthy 51 achievements are included.

This release was originally planned to be an episodic tower defense game, like their Classic game which features two episodes, however ignore the current iTunes description and game trailer, because the developers have scrapped that approach. Instead, they advise that a future update is likely, but not expected to require an in-app purchase.

If you already have the Classic version, you may not want this game unless you don’t mind repeating most of the levels, but otherwise 4Towers: Onslaught is a solid tower defense game which rewards intelligent play, has a lot of depth and improves on the aesthetics of its earlier version.