The ancient city of Nicaea and modern-day Iznik are enclosed along the eastern edge of Lake Iznik by about 5km (3 miles) of ancient city walls, made accessible through several ancient gates of which the Istanbul Kapisi is the best preserved. In the center of town are the well-preserved remains of the Church of Aya Sofya (admission 3TL), a roofless 11th-century church that served as the Patriarchate during the period of Byzantine exile following the fourth Crusade. The church preserves parts of the mosaic flooring and a partially exposed fresco of the Pantocrator in the niche of the left aisle. Excavations conducted in 1935, however, revealed traces of an older structure dating to the 6th century A.D. and attributed to Justinian.

Near the southwest corner of the church (across the street) are partially uncovered outdoor tile-production workshops from as early as the 15th century. Many of the brick and mud kilns are still intact.

The Yesil Camii dates to the late 14th century and displays a minaret covered with tiles in a colorful zigzag pattern. Unfortunately, these are not originals, as the actual tiles were destroyed. Across the street is the Nilüfer Hatun Imareti, built by Murat I and named after his mother, wife of Orhan Gazi and a Greek princess in her own right. Originally used as a charitable foundation and soup kitchen, the well-restored imaret (soup kitchen) now contains the Iznik Museum (admission 3TL), harboring a small collection of Roman and Byzantine artifacts and remnants from nearby burial mounds. There's also a small collection of Iznik tiles, as well as several ethnological items.

Thirty years of excavations have barely made a dent in the uncovering of the Roman Theatre, built by Pliny the Younger between A.D. 111 and A.D. 113 during his time as governor of Bythinia. Rather than build the theater into the side of a hill, the theater was constructed using vaults.