Birtvisi (Georgian: ბირთვისი) is a ruined medieval fortress in Kvemo Kartli, Georgia, nested within limestone cliffs in the Algeti river gorge. It is now within the boundaries of the Tetri-Tsqaro municipality, adjacent to the Algeti National Park, south-west of the nation's capital Tbilisi. Birtvisi is essentially a natural rocky fortress of 1 km², secured by walls and towers, the most prominent of which – known as Sheupovari ("Obstinate") – tops the tallest rock in the area. Various accessory structures, an aqueduct included, have also survived.