You are all forgetting about the Evil Eyes! After all, they formed the backbone of my army (as well as harpies!):smile23:
Eyes are eyes, but finding just one of each type on the Impossible difficulty can be much more difficult than finding a dozen of the same sulfur, and this is a huge disadvantage of these Eyes. And harpies are only good when upgraded, which also consumes resources at the beginning...
But overall, this castle is one of the strongest... and largely due to the selection of heroes in it.