Which school of magic do you prefer and why?

And how do you determine the number of griffons inside? Save-loading? Come on, that's not fair play.

You become a hero near the Conservatory but don't enter it. Then, you right-click and see who's guarding it. If it's a horde of griffons, it means there's one angel inside, and you can confidently go in. If it's a crowd, it means there are 2-4 angels, and it's risky to go in without checking, but in this case, you can either take a risk or check for 2500 gold by sending in a secondary hero. It's also possible that there's an upgraded stack, and you can determine that the Conservatory has 2 angels, meaning many royal griffons and a horde of regular ones.