@silkroad40: Like @for91days I highly recommend visiting the Yerebatan saray (the basalica cistern)! It is my favourite sightseeing place in Istanbul, because it's a unique and multidimensional mystical place with the ancient architecture and the light and sound effects. The atmosphere of the cistern was also featured in several films (e.g. James Bond), books and even in the video game Assassin's Creed: Revelations;-)