Süleymaniye Mosque is the second largest mosque in Istanbul. The eponymous sultan commissioned the mosque in 1550 and construction was completed in 1558. The famed Sinan Pasha, who also designed the Blue Mosque, was the lead architect. The mosque is located just north of the University of Istanbul, on one of the city's original seven hills.
The layout is based on Hagia Sophia, with a huge dome surrounded by minarets. The mosque's interior is 58m by 59m. At the center is the 27.5m diameter dome, which rises 53m, just shy of Hagia Sophia's. The interior is spectacularly adorned with intricate tilework and marble. The garden behind the mosque is the final resting place of Suleiman I (aka the Magnificent) and his wife Roxelana, as well as several other family members and a few lesser sultans. Across the street from the mosque to the north is the tomb of its architect, Sinan.
Getting to the mosque requires quite a bit of effort as it stands atop a steep hill, but it's definitely worth it. This is one of the world's great mosques. The view's nothing to scoff at either. It's a little difficult to access via public transportation, so you might want to take a taxi. Be warned that extensive renovations are ongoing, so some parts of the mosque or its grounds may be off limits.
Location: Süleymaniye Cadde, Eminönü
Hours: Open daily 09:30-16:30.
Admission: Free admission.