"Transfer into a host system at one of five trials sites and obliterate the crawler menace by detonating an energy bomb as quickly as possible. The host system is defenceless by itself but through the recovery of precious energy cells and scrap you can build and power various useful structures."
You control the wizard character who can walk freely, although slowly, around the selected level. Pressing the 'BUILD' key (Z or K) allows you to enter build mode, where you can cycle through the structures available to you and place them down onto the map. You can place towers that attack crawlers, ones that hide structures, a cap to place over nest holes, a teleport and the most important of all: the energy bomb.
Once you've put a structure down (and you'll need an attack tower to begin with) you must power it up by pressing L or X, as without power they won't do anything and the crawlers will simply destroy them.
With power, however, they'll unleash a barrage of death on the incoming nasties. When a crawler dies it drops energy cells, which you need to collect to power your defenses back up again. Also collect the corpses, as they are the currency you need to build more structures. As you progress through the waves different baddies arrive, some dropping power-ups like speed-ups. The aim, is to harvest enough cells to feed into the energy bomb so that it detonates.
There are several different levels to play through, plenty of nasty surprises in the form of mega crawlers, and great graphics and sound through-out. Well worth dedicating your gameplaying time to!