Really good job on the visuals, very coherent and atmospheric. The only thing that does not fit IMO is that a knight in a seemingly medieval setting has a gun at his disposal. The gameplay is simple and entertaining for a while but get's repetitive and is without an actual incentive. You could add, like other TD games do, harder enemies that take more hits until dead and some support like allies with crossbows in the castle's windows shooting at your foes, mages to slow enemies down, you name it. Being able to move the knight around and using a sword as a weapon also might things more interesting.