Fun game, really liked the dynamic.
The health upgrades for the tower are generally pretty useless, once the enemies have gotten close to the tower, even if you panic spawn knights, because they can't block the enemies path they'll still reach the tower. Even the max health of 45 disappears in an instant when there are 10 goblins hitting it at the same time.
I would sugest instead to replace the health upgrades with blockades that impede the monsters from advancing or maybe slows them down, like having to cross a moat.