Istanbul is a wonderful city and the only one that is divided over 2 continents, as the Bosphorus separates Asia from Europe. Most of the old city is in the European part and definatly worth a visit. The Topkapi museum, the Aya Sofia and the many huge Mosques are impressive, but also the spice Bazar and grand Bazar are the perfect places to realize that here 2 world cultures merge. Do not forget to eat a cheap fresh caught and grilled fish in the harbour
The Blue Mosque was nicknamed so due to the many beautiful blue mosaic tiles, though not much can be seen on this view. Entrance is free and though the Mosque is in use as a prayer room, also non-muslims are welcome and photos can be taken.
The city is a mix of modern western shops and clothing with traditional eastern culture. The sound of the calls for prayers in the morning makes you realize that Islam is the main religion here, though for me the sounds are just as curious as the Christian bells that wake me up elswhere in the world :-)