BEWARE Any bus or tram with a sign or indication that it heads for or calls at Topkapı will NOT take you anywhere near Topkapı Palace. Rather it will to a neighbourhood named after the city gate near which it’s situated (“cannon gate”), which may be worth a trip to take a look at the impressive city walls. Topkapı neighbourhood is located in the extreme western part of the old city, near the city walls, while Topkapı Palace is located in the extreme eastern part, which means the distance between each other is at least 7-8km. ‘Topkapı’ alone almost always refer to the neighbourhood, not the palace. Taking a bus heading for Topkapı to go to the palace is a common mistake made by visitors. For the palace, the stop/station you should look for is 'Sultanahmet'.
There are nice pieces of art and you will have a lot of fun if you like posing and taking pictures with all of the paintings. Otherwise you'd just walk by which doesn't take much time to see everything. Worth to come with friends and enjoy the place.
Get the audio necessarily as without it you won't understand much of the place. Important to visit once in Phnom Penh, gives you a big chunk of the history to think about.