Very fun game, I've always liked the tower defense genre and this game hit the spot.
But as the other comments pointed out, the lag needs some work. I believe what's causing the lag is the physics engine being overwhelmed by the amount of collisions happening between the instances of the 'Coins' when the enemy drops them. There is plenty of fixes for this: simplify the collision shape, turn off collision for new instances over a certain amount, add tiered coin amounts so less coins are colliding, etc...
Anyways, this game has good potential. Keep up the good work.