The Medieval citadel standing beside the old town
This was the capital of bulgaria for a long time based almost purely on the geography of the place.
1) It's near the centre of the country,
2)The citadel could be built on an isolated, almost vertical sided hill. It's within a huge loop of a meandering river. A fluke of nature has left a 100m long, 4m wide raised walkway that connects the main street of the town to the citadel gates.