I've just released Version 0.41. Main changes are:
UI update
Onboarding improvement / simplification
Enemy targeting improvements - you can now apply selected targeting to all towers, or all towers of the same type
Sniper: 10% extra damage each time a given sniper tower hits the same enemy
Cracked tiles: if you delete tiles on the same grid location 4 times or more you will no longer be able to build on top of them.
Starting the next wave early now doesn't cost you time. E.g. if you start the wave 5 seconds early your mana bar will refill as if those 5 seconds had occurred rather than been skipped.
Right click now rotates your tetrominoes
Option to remove damage numbers (should help performance at excessively high wave numbers!)
Option to disable the slow-down while placing a tetromino