The tower defence genre has had something of a rebirth on Apple's handheld, undeniably due to the tactile nature of dragging and dropping towers via the touch screen. A quick flick through the App Store in search of TD games could pit you against marauding pirates or lurching zombies. In the case of geoDefense, it's killer shapes.

geoDefense's stages feature killer shapes following a path that terminates at the player's home base (denoted by a number signifying how many lives the base has). It's up to the player to situate various different towers at strategic points along the way to try and destroy the waves of enemies before they reach their goal. Each different tower in geoDefense can be upgraded and the real strategy comes from placing the right tower in the right place; the laser towers operate best at the end of straight segments while the blaster tower does most damage when placed inside corners.

There's an almost mathematical approach required when tackling some of the harder stages in geoDefense. Working out where to place laser towers so that they cross over, or bunching enemy-slowing stun towers around crucial intersections becomes a myriad of trial and error gameplay and pure focused thinking. Even on the easy settings geoDefense is a hard game, and it's guaranteed to test the patience of most seasoned tower defence veterans.

geoDefense's graphical style is torn straight from the Geometry Wars book of visuals, with levels starting out with a lifeless grid that soon bursts into colour and fluidity whenever an explosion occurs. The two games are so different in style of gameplay that geoDefense escapes the clutches of poor imitation and stands out as a good-looking game in its own right.

It won't revolutionise the tower defence genre, but it's certainly one of the nicest looking examples available for the iPhone. Its main downfall comes from just how difficult some of the later stages become. This isn't a game for the easily discouraged, but if you're willing to put in the time and effort geoDefense delivers some truly satisfying gameplay.