1 Kerim Bey: Come down into the sewers with me, James.
1 Stud: Er... okay.
2 {scene change: a vast underground cistern with columns and arches}
2 Kerim Bey: The Emperor Constantine built this as a reservoir sixteen hundred years ago.
3 Kerim Bey: This way we can get under the Russian Embassy and spy on what they are doing.
4 Stud: Oh! I thought you were just showing me the scenic delights of Istanbul.
4 Kerim Bey: That too.

James Bond films are well known for using glamorous locations for filming otherwise mundane scenes.

The film location used in the movie is the Basilica Cistern, which was indeed constructed by the Emperor Constantine 1600 years ago, although it was significantly altered and enlarged by Justinian a couple of hundred years later. It's a cathedral-sized underground space, filled with columns carved in intricate designs and a beautiful vaulted ceiling.

So it's actually a bit more than a mere sewer.