Enemies are the main thing you fight in the game. There are many types, the stronger ones usually being slowest. Here's a list of enemies...
|Bluezer||2||Round 4||2||$2 ($3
|Greenzer||3||Round 8||1.5||$3 ($6 including
|Yellzer||4||Round 12||1||$4 ($10 including
|Goldie||1||Every 10-20 seconds||Moves away from
the player's aim
|Contaminated Redzer||2||Round 20||A tenth of the player's health||$5 ($6 including children)||5|
The amount of enemies spawned each round depends on what type it is. Here's the formula for how many will be spawned each round:
n = r / f
n = number of that enemy type spawned on this round
r = round number
f = first round it appears on
The exceptions to this are the Goldzer.