Nice game! As it stands, later 'enemies' appear higher than more recent (and therefore more imminent) ones. The downside is that you sometimes can't read a number properly because it's occluded—with that font, the bottom of a character is not sufficient to identify what the character is.
My suggestion is to have the enemy z-index start at a high number and decrement per enemy (eg, z-index = 1000 - enemy_number). Trivial fix, and that way more pertinent enemies are on top with their numbers visible.
My suggestion is to have the enemy z-index start at a high number and decrement per enemy (eg, z-index = 1000 - enemy_number). Trivial fix, and that way more pertinent enemies are on top with their numbers visible.