Enjoyable, but not a great take on this kind of game.
When you reach the level goal it increases enemy HP faster than the Gold they give. This makes it better to end a level with 1 enemy left than go to the next level before you're loaded with upgrades, you can get a reliable 14 coin at level 2 and drop to 27 because you reached level 3. This means the Timer upgrades actually hurt you, they force you to wait longer before the level resets.