Thank you for playing and a valuable feedback.
I'll take a look at the button collisions.
I am thinking to add priority system but I don't know how implement if well.
I'll try to make change in difficulty more gradual.
yes the lack of necessity to upgrade is a known issue, I'll rebalance the game for the next update.
I don't know GameMaker, so I'm not sure if this will be of any help. When we did a tower defense game, we had each tower store an array of enemies that were in range, updating as they entered and exited. When firing, or each frame, the tower would loop through the array and pick the best target based on the stat that was chosen for targeting.
