Having to do your best with random towers is a cool idea, but there are a few things that are stopping it from shining. Firstly, the enemies seem to spawn at the same rate all the time, so there is no way to succeed at the start and no way to fail once you have 3 or so towers. Secondly, the player needs to know what the different towers do, this will let them get excited about each 'roll'. Finally, since the only decision (beyond the obvious, push buy, option) is tower placement, this should matter a lot. Maybe have towers that only shoot in one direction or have wildly different ranges (e.g. a tower that can shoot at long range but not close range)?

Basically it's a tower defense but no strategy involved