Often overlooked for its glorious Classical past, Athens offers a unique perspective into Ottoman architecture. The Fetiye Tzami is located in the Roman Agora. Until 2010 it housed artifacts from the Roman Agora and nearby excavations. Although never fully restored to its 15th century state some floriated frescoes are preserved on its ceiling. The remains of a minaret are preserved also. To get an idea of the interior of a tzami visit the nearby 18th century Tsisdarakis Tzami near the Monastiraki subway station.
Be sure to take the time to look at the modern Arabic graffiti. I was lucky enough to gain access via special permission of the local curator, but access to the interior is usually prohibited. Ottoman buildings often reused materials from Classical buildings (spolia), so be sure to look out for column capitals that look older than their present location. Behind the mosque also lies many Ottoman gravestones. Take the time to explore and you will find that there is more to Athens than gyros!