|Basilica Cistern (Source: Wikipedia)|
The Basilica Cistern (pictured above) was built in Constantinople in the 6th century by the Byzantine Emperor, Justinian I. It is a huge underground structure. Check out the dimensions (according to Wikipedia): This cathedral-size cistern is an underground chamber approximately 138 metres (453 ft) by 64.6 metres (212 ft) - about 9,800 square metres (105,000 sq ft) in area - capable of holding 80,000 cubic metres (2,800,000 cu ft) of water. The ceiling is supported by a forest of 336 marble columns, each 9 metres (30 ft) high, arranged in 12 rows of 28 columns each spaced 4.9 metres (16 ft) apart.
For more info on the Basilica Cistern, click here and here.
|Theodosius Cistern (Source: Wikipedia)|
The Theodosius Cistern (pictured above) dates back to about 430 A.D. It is smaller than the Basilica Cistern, but it still an impressive 150 feet by 80 feet, with columns towering at around 30 feet high. Click here for more info.
|Cistern of Philoxenos (Source: Wikipedia)|
This was an older cistern, built under a palace and restored by Justinian I in the 6th century. Called Binbirdirek in Turkish ("Binbirdirek"="1001 columns"), this cistern actually has 224 columns, each about 50 feet tall. Click here for more info.
Cisterns as Dungeons
It seems to me that most medieval urban RPG dungeons are sewers or catacombs, but using cisterns for at least part of an undercity dungeon adds some additional flavor. If the cisterns are active, it also forces player characters to contend with water and water-bourne critters.