The main Crusader army, largely French and Norman knights under baronial leadership, had begun to cross over into Asia Minor from Constantinople at the end of April 1097.

Godfrey of Bouillon had been the first to arrive at Nicaea, with Bohemond of Taranto, Bohemond's nephew Tancred, Raymond IV of Toulouse, and Robert II of Flanders following him, along with Peter the Hermit and some of the survivors of the People's Crusade, and a small imperial force under Manuel Boutoumites and Tatikios.

They arrive on May 6, severely short on food, but Bohemond arranges for food to be brought by land and by sea.

They put the city to siege beginning on May 14, assigning their forces to different sections of the walls, which are well-defended with two hundred towers.

Bohemond camps on the north side of the city, Godfrey on the east, and Raymond and Adhemar of Le Puy on the south.

Arslan is away campaigning against the Danishmends in central Anatolia at the time, and has left behind his treasury and his family, underestimating the strength of these new crusaders.

On May 16, the Turkish defenders sally out to attack the crusaders, but the Turks are defeated in a skirmish with the loss of two hundred men.

The Turks send messages to Kilij Arslan begging him to return, and when he realizes the strength of the crusaders he quickly turns back.

An advance party is defeated by troops under Raymond and Robert of Flanders on May 20, and on May 21, the crusader army defeats Kilij in a pitched battle that lasts long into the night.

Losses are heavy on both sides but in the end the Sultan retreats, despite the pleas of the Nicaean Turks.

The rest of the crusaders arrive throughout the rest of May, with Robert Curthose (accompanied by Ralph de Guader) and Stephen of Blois arriving at the beginning of June.

Meanwhile Raymond and Adhemar build a large siege engine, which is rolled up to the Gonatas Tower in order to engage the defenders on the walls while miners mine the tower from below.

The tower is damaged but no further progress is made.

The siege continues, but the crusaders have little success as they find they cannot blockade the lake, upon which the city is situated, and from which it can be provisioned.