Alaaddin Mosque is one of Niğde's finest Seljuk buildings, built in the 13th century below the castle. Its great pleasure is the stonework: geometric carving, a strong portal, and the sober proportions of a mosque built for a frontier Anatolian city rather than an imperial capital.
It is still a working mosque, so visit outside prayer times and dress with the usual respect. The building rewards a slow look at the entrance before you step inside.