Church-to-mosque-to-church: Agios Titos, the Church of St. Titus in Heraklion, Crete (photos from 2 years ago). Its modesty belies a bewildering story! Previously a Venetian Catholic church, the present edifice of 1869 was an Ottoman mosque designed by a Greek Orthodox architect. 
It's a complex history. This city was the new Muslim capital established by Andalusī Arabs who conquered the island from Byzantines in the early 9th century CE. It was called Khandaq, after their battle "trench" (Image: Saracens sailing to Crete, from the Madrid Skylitzes MS)
A church dedicated to Crete's patron saint was built in the city ("Chandax") sometime after the Byzantine reconquest of 961 CE. The earlier 6th century basilica of St. Titus still stood in the original Roman capital Gortyn, but the saint's relics were eventually transferred.
Following the sack of Constantinople in 1204, Crete was purchased from the Crusaders by Venetians. The cathedral of St. Titus remained a landmark of the city ("Candia"), rebuilt multiple times after earthquakes and fires. Though Catholic, it shared space with the Orthodox.
The Ottomans took the city in 1669 after one of the longest sieges ever (21 years). This was the last conquest in Ottoman history. The relics of St. Titus were removed to Venice, and the church was turned into a mosque by the Köprülü grand vizier, Fazıl Ahmet Paşa.
The so-called Vizier Mosque was destroyed by an earthquake in 1856. A new mosque was commissioned by grand vizier Ali Pasha in 1869.
The mosque's architect was Athanasios Moussis, from Istanbul's historically Christian neighborhood of Tatavla (today's Kurtuluş, Şişli) with family origins in Epirus in mainland Greece.
After the Greek-Turkish population exchange of 1923 that had Cretan Muslims exiled from the island and resettled throughout the Middle East, the minaret was demolished and the mosque was turned into a church in 1925.
The skull of St. Titus was returned from Venice to Heraklion in 1966, and is now held in a reliquary inside the church.
Looking at the back of the building as it stands today, I couldn't quite decide if that looked like a mihrab or an apse!
