There's a point around wave 35 where the zeds don't come in significant numbers anymore. Everything bought and upgraded and the gameplay turns to sending the folk to the left side, you go standing on the right barrier with the upgraded sniper rifle and just press the click aiming at their heads.

Before that point, though, it was really intense and you really had to question if to repair or buy the new gun where to upgrade the characters. So have 4 stars for all the great tension building up, but once you are upgraded and all set up, there's no more gameplay.