Many a tourist in Istanbul has experienced “mosque-fatigue.” This common but preventable condition is usually the result of visiting too many mosques in too little time, without first educating oneself about their distinguishing architectural features—the very things that make a visit to a mosque rewarding and stimulating.

The Küçük Aya Sofya Camii, or Little Hagia Sophia Mosque, just west of Cankurtaran, is not the only mosque or ex-mosque in Istanbul to have once been a Christian church. The same is true of the Arap Camii in Karaköy, and of course of the larger and more famous Hagia Sophia itself, now a museum. (The Church of the Chora likewise underwent the same transition from church to mosque to museum.) Nonetheless, the fact that Little Hagia Sophia is still a working mosque makes the visible remnants of its Byzantine heritage all the more striking.

Like the Sokollu Mehmet Paşa Camii around the corner in Kadırga, Little Hagia Sophia is still relatively unknown to foreigners. Lying slightly off the beaten tourist track, in a working-class neighborhood known as Küçük Ayasofya Mahallesi, the mosque reached its nadir in the early-to-mid nineties, when the grounds were in such a poor state that a local newspaper complained about the beer cans and other refuse lying around the courtyard. Since then, extensive restorations have taken place, and Little Hagia Sophia is now open to both worshipers and visitors.

A small sign at the entrance to the mosque grounds tells the story of Little Hagia Sophia – formerly known as the Church of Saints Sergius and Bacchus – in both Turkish and English. According to the sign, in the early 6th century Byzantine Empire the young Justinian (later to become emperor and build the larger Hagia Sophia) was going to be put to death for participating in a revolt against his uncle, the Emperor Justin. (More authoritative scholarly sources identify the emperor in question as Anastasius, Justin’s predecessor.) St. Sergius and St. Bacchus then appeared to the emperor in a dream, urging him to spare Justinian’s life. In gratitude, Justinian built the church, dedicated to the two saints, shortly after becoming emperor himself in 527.

Your first clue that this was originally a Byzantine church is the material of which it is built. While most Ottoman mosques are built out of stone, Little Hagia Sophia is made of a mixture of brick and mortar in which (as often in Byzantine churches) the long thin bricks seem to serve as dividers for the mortar rather than vice versa. Unlike Hagia Sophia’s circular dome, that of Little Hagia Sophia resembles a shallow inverted soup bowl with a flared rim. Next to the mosque is a low, squat minaret; there is a tomb on the left, plus a small Ottoman-era graveyard running around the rear of the building.

Those who have visited the Sultanahmet Mosque as tourists will find the protocol far more relaxed here – if you come just after the ezan has sounded, simply wait 5-10 minutes on the benches in front, till the small number of worshippers exit.

Once you enter through the domed portico, you’ll also be struck by the simplicity of the interior. Whatever its original state in the 6^th century (which, to judge from the comments of the Byzantine historian Procopius, was apparently as dazzling as the mosaics of the larger Hagia Sophia), the interior surface of Little Hagia Sophia is mostly – save for the designs above the vaults, and around the windows – a plain white. Small circular paintings on the sections of the dome are emblazoned with the names of Allah, Muhammad, and the first caliphs, in standard fashion.

There are two things in particular that make Little Hagia Sophia unique among Istanbul mosques. The first is the seven pairs of beautiful columns both on the main floor and on the gallery (there are also two extra, asymmetrically placed columns on the lower level, and four above.) The shafts of the columns are made of maculated colored marble; the white capitals are even more distinctive, with perfunctory Ionic scrolls beneath elaborately carved floral patterns, from a distance somehow resembling pebbles hollowed out by sea worms or mollusks.

Another, perhaps less obvious, feature is the dedicatory inscription to St. Sergius in Greek running along the entablature of the columns. In contrast with the mosque’s brightly-colored Arabic calligraphy, this low-relief inscription, made of the same white marble as its entablature, would be easy to miss on a cursory visit to Little Hagia Sophia.

The presence of this inscription is, when you think about it, incredible. Four to five times a day, a group of worshipers perform Muslim prayers in Little Hagia Sophia – while above their heads runs a still-legible dedication to a Christian saint. Though it would be too facile to tout this as an example of Turkish religious tolerance, such a juxtaposition could nonetheless serve as a symbol of Anatolian civilization over the past few millennia: a Turco-Islamic overlay above a Hellenic-Christian past.

In front of the mosque there is a courtyard, in the center of which is fountain for ritual ablutions. The courtyard is surrounded by small cells that originally served as dervish lodges, then as a medrese (religious school), and now house artisans and second-hand booksellers. Here you can buy handmade ceramics, as well as exquisite original Ottoman-era illuminated manuscripts.

If you are not operating under severe time constraints, or if you have already seen the major sights of Istanbul and want to visit somewhere off the beaten path, a visit to Little Hagia Sophia is strongly recommended.