Click to place down a tower, and use the scrollwheel (or up/down arrow) for switching characters! Now, you see a transparent square around the mouse-pointer... it shows the range of the tower! Press "E" while hovering the mouse over a tower to upgrade it! (if you have the money to do it!) Press space to start the match (as the popup says at the start). As you continue to play, the older towers age... When a tower is older than 3 waves, it gets removed! So you have to replace the old towers with new ones! Eliminate 9 different enemies to win! (after 4 enemies, the enemies level up!)
Thanks to @Makoy12172009 for the art! Ask them if they can make you a character, and I might add you to the game! Thanks to epicinsar for the motivation to add a better attack system, and add a visual projectile! (he now also has his own tower that deals 1000 damage, and never ages!)