Ananuri is a fortress on the River Aragvi. It was in use until the 19th century. It consists of two castles, one of which is only in ruins. The 17th century church contains the remains of some frescoes, most of them have been destroyed in a fire in the 18th century though. What is left is pretty amazing though.
You pass it when you travel on the Georgian Military Road.
It is currently on the tentative list to be included as a UNESCO World Heritage site.