Pantanassa Monastiraki 

When you walk at the Monastiraki square, you will notice close to Athinas street a beautiful building, that is the Church of the Pantanassa  or of the Dormition of the Theotokos (Koimisi tis Theotokoy in Greek), a 10th-11th century old Orthodox church of a previous larger establishment, from which actually Monastiraki took its name from. 

Tzistarakis Mosque 

At the same square there is also the famous previous mosque, Tzistarakis, now a museum. From there, by walking up Pandrosou, you can enter Plaka area and explore its museums and rest of Orthodox churches.