The wizard heard Julian trying to open the gate, thanks to his acute hearing and Julian's bad dice roll! Since he was up to no good, he responded by immediately casting a spell at you. It's too bad that Mortus's fireball went nuclear, but these things happen sometimes.
Rufus, does the building show signs of inhabitants? Light on or anything? Is it a dark room?. I'm assuming it appears vacant in my post.
I wrote my post in a bit of a hurry. It looks dark and empty, as far as you can see.
Mortus and Julian need to say which way they're going next.