Ivan of Rila needed time to himself back in the middle of the tenth century, so he became a hermit in a mountain cave near Rila. That attracted acolytes and soon the cave became a little crowded so they founded a monastery nearby. In the 19th century this grew into a huge complex with a beautifully painted central church, conveniently located right next to a large bus parking lot, so no more hermits. It was built for 300 monks; today there are six.
We also visited Boyana Church close to Sofia. Also built in the tenth century, but still with the original paintings covering the walls and vaults. You totally feel the millennium since its construction. No photos allowed unfortunately.