Right now I'm not very satisfied with the endgame. Last thing I did there was adding the armor concept, but it didn't make it much more challenging. I think it is about time to add a new factor to the creep hitpoint formula that kicks in at around wave 140.
Finished an experimental endgame hp factor (exp(sqrt(x))). At least from the function plot, it boost the endgame significantly. I need to give it a few testrounds first, however. But it feels good so far. Thanks Vasuhn for pushing a little bit in that direction.