Ghast

Ghasts float over any trap, so you should upgrade your turrets to stop them. They come out on waves which have the unit value of 5, such as 5, 15, 25 and 35.

The number of ghasts appears to follow the pattern of the next practical number value, based on the number of waves which ghasts have previously spawned on. Examples of practical numbers are 1, 2, 4 and 6.