I think the game could use some more animations/visual feedback so that the player knows that things are working.
E.g.
-The enemy can flash red when hit by a unit
-Arrow projectile for the archer + a hit animation when in contact with an enemy
-Sword swing animation/radius for knight and patroller.
It would be nice if you could switch units via the onscreen UI/buttons (I kept accidentally pressing the buttons and spawning a unit underneath the button). Also I accidentally spawn enemies inside the walls.
But good job on the rest of the game, it's a bit slow but the mechanics work :)