This is the Mama Hatun Kümbet, a grave monument for a Seljuk princess. She had supported Saladins conquests in these areas with her troops. It was built by Sesi Muffada ("the cross-eyed"), a prince from Ahlat at the Van-lake. It consists of the actual grave (türbe) and a circular wall around it. It was closed, but I took several shots through the grate at the entrance.
This is a detail of the türbe itself.