For those tired of the same old “zombie game” here are a few more enemy types:  demons (imps shown), aliens (basic alien shown firing a laser), and robots (not shown).  Each of the enemy types have at least 3 variations (with a few exceptions, like the sludges) and a boss for each enemy variation.  Each enemy type also has a “spawner” associated with it that will endlessly spawn enemies until they are destroyed.  Good luck against the hordes of creatures that will be thrown at you!