Topkapi Palace was initially built in 1460 by Sultan Mehmed II in Istanbul. This palace was once the main residence of the Sultan in power as well as his family and concubines. It was also the administrative and educational centre of the state. The Palace was continually built onto and expanded over time. To visit Topkapi Palace, you need to give yourself a good half a day as there is so much to see within the walls. Be sure to pay extra to go and see the magnificent rooms housed within the Harem, which is where the many wives and concubines of the Sultans resided. The exquisite and refunded tiles in some of these rooms are worth every cent of the entrance fee. There are numerous courtyards and extensive palace grounds to wander through and you can try to start to imagine the secrets these walls hold over an enormous period of time. The Palace also houses some ancient relics from the Ottoman Empire, showcasing some of the wealth they had in displaying some magnificent jewels. It also houses some ancient relics of The Prophet Mohammed.
Other major sites that should be on your all lists of things to do in Istanbul are the Ciragan Palace, Kucuksu Palace and Yildiz Palace.