The Dolmabahce Palace was built in the 19th century, right on the shore of the Bosphorus, and became the home of the sultans who then left Topkapi Palace behind. It is far more European-looking than Topkapi and is incredibly luxuriously decorated with crystal banisters and at least one chandelier whose size is measured in pounds, not inches. Huge, elaborate carpets are carefully protected from wear, and the furnishings are very ornate and surprisingly European.
You must take a guided tour to visit the palace, and the cost is about $5. You enter the gate near the clock tower, and at least on the day we were there, we were entertained while waiting in line by a soldier holding PERFECTLY still. Once every half hour, another soldier would approach him, whisper in his ear, close his eyelids, and massage his legs--with no visible reaction from the first soldier. We never learned whether this is usual, or some sort of special event, or even a punishment (that would be my vote!) but it did serve to keep us occupied while we waited our turn for a tour. We didn't even catch him blinking!
You should expect a fairly long wait for your tour, an hour or more--and the tour itself takes about an hour and a half. Also, expect to have your camera confiscated; you can pay a fee to take your camera into the palace, but not if it has a flash, and without a flash most of the palace is much too dark for picture-taking.
The palace itself is incredibly opulent, but in some ways not as interesting as Topkapi Palace because Dolmabahce shows so much European influence. There is still a harem, of course, but overall it is not as exotic here. And the 20th century is evident in odd touches--all the clocks are stopped at 9:05, when Ataturk died here in 1938.
We really enjoyed our visit to Dolmabahce, and were astounded by the contrast between it and Topkapi, inhabited by the sultans until Dolmabahce was built. What a shock it must have been for anyone familiar with the exoticism of Topkapi to come to this new palace, whether to live or simply to visit!