Short answers, pulled from the story.
Diocletian designated Nicomedia the eastern and most senior capital of the Roman Empire in 286 AD because of its strategic location at the crossroads of roads leading deep into Asia Minor. He then introduced the Tetrarchy system in 293, and Nicomedia held its senior capital status until 324 AD.
On the 23rd of February 303 AD, the festival of the Terminalia, Diocletian ordered the newly built Christian church at Nicomedia razed, its scriptures burned, and its precious stones seized. The following day he issued his First Edict Against the Christians, extending these measures across the Empire. Fires destroyed part of the imperial palace shortly after, and Galerius oversaw multiple executions of those he blamed for the fires.
Constantine used Nicomedia as his interim capital for six years after defeating Licinius at the Battle of Chrysopolis in 324 AD. In 330 AD he declared the nearby city of Byzantium, renamed Constantinople, the new capital of the Roman Empire. Constantine died in 337 in a royal villa near Nicomedia.
Nicomedia was founded as a Megarian colony in 712-711 BC under the name Astacus, meaning lobster. After being destroyed by Lysimachus, it was rebuilt in 264 BC by Nicomedes I of Bithynia and renamed Nicomedia. It then became the metropolis and capital of the Roman province of Bithynia.
Nicomedia was first besieged by the Ottomans in 1304 and again in 1330, before finally falling to them in 1337. The Byzantines briefly retook it following the Battle of Ankara, but it passed definitively into Ottoman hands in 1419.
The 1999 İzmit earthquake led to the discovery of ancient statuary including figures of Hercules, Athena, Diocletian, and Constantine during debris clearing. A more extensive excavation of Diocletian's Palace began in April 2016 under the Kocaeli Museum, which estimated the palace site covers 60,000 square meters. Other surviving traces include a second-century AD marble nymphaeum, a large cistern, and fragments of the harbor wall.