New version is up. Changes:

Playing field is now circular, compared to being squared before. Feedback suggested that it was too easy to hide in the corners, so now - there aren’t any!
Bouncers have been reduced in size from 9 units to 7 units.
Starting layout of enemies changed slightly.

Coming up next:

A third enemy - The Sleeper! (as suggested by @PirovanoMichele )
New difficulty curve - start with fewer enemies and spawn in at a steady rate (as suggested by GoodReactions)