if you talk to him what does he say? Are you playing an evil character? If not, you just need to walk in between the two columns that are beside him.
he says something along the lines of, "you are not ready to enter the place" im tryin to find that werewolf cure for the bunny farmers daughter. i have an alignment of +78.
You must speak to Nasrudin first. Then go there. He will say that the diviner had sensed u are coming and will welcome you inside. DO NOT KILL THE GUARD IF YOU CANT GO IN. If you kill him you will become enemy of Tulla. If you kill 1, only 1 person in Tulla, all the wizards are after you.
Don't kill em before you have figured out how to get to the vendigroth ruins, or you'll not be able to finish the game...