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 Tetritsqaro Municipality, adjacent to the Algeti National Park, south-west of the nation's capital Tbilisi.
|Location||Kvemo Kartli, Georgia|
|Coordinates||41°36′32″N 44°32′21″E / 41.60889°N 44.53917°E|
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.
In written sources, Birtvisi is first mentioned as a possession of the Arab amir of Tiflis of which he was divested by the Georgian nobles Liparit, Duke of Kldekari and Ivane Abazasdze in 1038. In medieval Georgia, Birtvisi entertained a reputation of an impregnable stronghold whose master could control the entire strategic Algeti gorge. The Turco-Mongol amir Timur notably reduced the fortress during one of his invasions of Georgia in 1403. After the partition of the Kingdom of Georgia later in the 15th century, Birtvisi was within the borders of the Kingdom of Kartli and in possession of the princes Baratashvili.  Birtvisi Fortress is not studied archeologically. 
- ^ Protected Areas: Algeti National Park Archived 2011-07-21 at the Wayback Machine. Agency of Protected Areas of Georgia. Accessed June 18, 2011.
- ^ a b (in Georgian) ზაქარაია პ. საქართველოს ციხე-ქალაქები, ციხესიმაგრეები, ციხე-დარბაზები, ციხე-გალავნები, კოშკები.-თბ.,2001.-გვ.86-88.
- ^ Thomson, Robert W. (1996), Rewriting Caucasian History, p. 289. Oxford University Press, ISBN 0-19-826373-2
- ^ Allen, William Edward David (1932), A History of the Georgian People: From the Beginning Down to the Russian Conquest in the Nineteenth Century, p. 125. Taylor & Francis
- ^ (in Georgian) ბირთვისის ციხე
- ^ (in Georgian) ბირთვისი