The great military commander Hannibal Barca came to Nicomedia in his final years and committed suicide in nearby Libyssa (Diliskelesi, Gebze).

[5] Pliny, in his letters, mentions several public buildings of the city such as a senate-house, an aqueduct, a forum, a temple of Cybele, and others, and speaks of a great fire, during which the place suffered much.

On 23 February 303 AD, the pagan festival of the Terminalia, Diocletian ordered that the newly built church at Nicomedia be razed, its scriptures burnt, and its precious stones seized.

[8] Nicomedia remained as the eastern (and most senior) capital of the Roman Empire until co-emperor Licinius was defeated by Constantine the Great at the Battle of Chrysopolis (Üsküdar) in 324.

Situated on the roads leading to the capital, the city remained a major military center, playing an important role in the Byzantine campaigns against the Caliphate.

By that time, most of the old, seawards city had been abandoned and is described by the Persian geographer Ibn Khurdadhbih as lying in ruins, with settlement restricted to the hilltop citadel.

The city was briefly held by the Latin Empire following the fall of Constantinople to the Fourth Crusade in 1204: in late 1206 the seneschal Thierry de Loos made it his base, converting the church of Saint Sophia into a fortress; however, the Crusader stronghold was subjected to constant raids by the Emperor of Nicaea Theodore I Laskaris, during which de Loos was captured by Nicaean soldiers; by the summer of 1207 Emperor Henry of Flanders agreed to evacuate Nicomedia in exchange for de Loos and other prisoners Emperor Theodore held.

Pliny the Younger complains in his Epistulae to Trajan, written in 110 AD, that the Nicomedians wasted 3,318,000 sesterces on an unfinished aqueduct which twice ran into engineering troubles.

[5] The ruins of Nicomedia are buried beneath the densely populated modern city of İzmit, which has largely obstructed comprehensive excavation.

Other monuments include the foundations of a 2nd-century AD marble nymphaeum on İstanbul street, a large cistern in the city's Jewish cemetery, and parts of the harbor wall.

[5] The 1999 İzmit earthquake, which seriously damaged most of the city, also led to major discoveries of ancient Nicomedia during the subsequent debris clearing.

[23] In the years after the earthquake, the Izmit Provincial Cultural Directorate appropriated small areas for excavation, including the site identified as Diocletian's Palace and a nearby Roman theatre.