I've noticed that at difficulty 3, it simply turns into vampire spam, causing my game to lag from the number of units.
Yeah I know the goal is to complete the summoning ritual for each level, but sometimes I take too long to do it, and the turrets can't beat the super chonky guys fast enough so I end up making a huge wave of vampires... then I remember oh yeah the summoning ritual XD.
500 vampires is pretty funny though.