Love the idea, one immediate improvement that could be made would be to spawn multiple units easily. Perhaps an input field or drop down that will let you select how many units at once you wish to place, or by placing multiple units if you drag your mouse with the button held down.
Also more information about the troll's hp and the tower's damage would be great.
I plan to test out features such as damage indications, some particles or other methods of display attacking effects and so on, for the placing multiple enemies at once I did have an idea for spawning in squad formations, you could drag a selection box for the amount of units but obviously that was a bit too ambitious for the Jam haha, I may come back to that idea though.
Thank you for playing :)
