This is a minor entrance to the Mihrimah Sultan Mosque, also called the Iskele Camii (iskele is quay or jetty, it's the mosque facing the spot where the ferries to Istanbul make land). In was built in 1547-8 by Sinan for Mihrimah Sultan, daughter of Süleyman the Magnificent and wife of the Grand Vezir Rüstem Pasha (between the two of them they built some marvellous mosques).

The building to the left houses a modern clinic now. You will see it one one of the next pictures.