This mosque is situated inside the Roman Agora site. The inside is not accessible.
It is said to be constructed by Mehmet II in 1456 to commemorate the capture of Athens. It is, however, possible that he did not really construct it, but converted a preexistant Christian church. There remains some Muslim tombstones as you see in the photo left.
The photo below is the mosque seen from behind; from this angle, it really looks like a Byzantine church.
On the other side of the street in front there is gate of the Madrassa, or Islamic theological school, and a Western visitor recorded that some Dervishes were using the Tower of the Wind. It is probable that the area was one of the islamic religious centres in Athens under the Ottomans. Also close is the Tzistaraki Mosque in front of the Monastiraki metro station.