Can anyone explain me what happened here???

Did you destroy a tower at the same time you destroy the castle gate? This could explain the problem.

I destroyed everything. I went to the gate with a huge army, elite ogre and cannons. I think it was a bug. Saddly I only got almost half of the skulls because of this.

http://forums.flaregames.com/topic/7232-2-crowns-and-91-after-clearing-100-of-the-path/#entry49418

It’s a bug