It's "Stürmt die Burg!" (in English: "Storm the castle!")
It's an inside joke going back to our early days of playing AD&D 1e when the party encountered a castle that had been taken over by giants. The castle was blocking a mountain pass, and since one pf the players was fed up discussing how to proceed, his character just charged at the castle with this battlecry. The others sighed, but decided to charge after him. Naturally, they were pelted by rocks, killing off the party's Paladin, but surprisingly they actually managed to take the castle.
