The Basilica Cistern is one of Istanbul's most surprising tourist attractions. Once stored the imperial water supply for the Byzantine emperors. The project was begun by Constantine the Great but finished by Emperor Justinian in the 6th century. Many of the columns used in construction were recycled from earlier classical structures so feature decorative carvings.
The most famous of these are the column bases known as the Medusa stones in the northwest corner with their Medusa head carvings. Around the cistern, you may see The Hagia Sophia, The Blue Mosque, and Hippodrome. The cistern is 5 minutes walk away from Eresin Hotels Sultanahmet.