Ultratron (2005)

Chasers, turrets and spawners give you reasons to move to different parts of one compact arena instead of tracing a single safe circuit. Shops between waves let you strengthen the droid and buy supporting pets, adding a little planning without turning the game into a long exploration map. The PC release works with independent movement and aiming. Waves are quick units, while the forty-plus-level sequence is a larger undertaking. Its glowing effects can hide a small threat when several systems fire together.