West come to East

Western dukes of First Crusade,
 from left Godfrey de Bouillon,
Bohemond I of Tarent, 
Raymond IV de Toulouse,
Tancred of Hauteville
Source: wikimedia commons

1097 AD - 2 years after the famous sermon was given by Pope Urban II in Council of Clermont call for Crusades - western knights arrive at Constantinople gates. Between them there four noble dukes: 

• Godfrey de Bouillon - second son of the count of Boulogne, with blood lineage back to Charlemagne. Born around 1060 AD in Boulogne-sur-Mer . Describe by medieval chronicler William of Tyre as taller than 'the average man ... strong beyond compare, with solidly built limbs and stalwart chest, [with] pleasing features  [and] beard and hair of medium blond'.
• Bohemond of Tarent, son of famous Robert Guiscard (the Astute), Duke of Apulia and Calabria
• Raymond IV of Toulouse - count of Toulouse, duke of Narbonne and margrave of Narbonne.
• Tancred of Hauteville, grandson of Robert Guiscard so also the nephew of Bohemond.

Emperor Alexius I Comnenus, although ask for support, was surprised by the size of army which arrived at his gates. He envisioned that will come an organised team of knights not a group of proud and ambitious dukes with their armies. 

"Having requested aid from Pope Urban in 1095, he was now confronted by a swarm of Latin crusaders. But for all their supposed unruly savagery, he recognized that the Franks’ brutish vitality might be harnessed in the interests of the empire." Source: Asbridge, T. "The Crusades: The Authoritative History of the War for the Holy Land", Kindle edition, p. 49

Godfrey de Bouillon made the oath to Alexius I Comnenus
Source: wikimedia commons

Crusades expected that emperor will strengthen them with his army, but that's not happened - instead he plays the usual game of powerful rulers - divide and rule - he invites to his palaces knights one by one. Start from Godfrey de Bouillon who was invited on 20 January 1097 to opulent Palace of the Blachernae. The way how he was welcome was intended to address emperor goals.

"Godfrey apparently found the emperor ‘seated, as was his custom, looking powerful on the throne of his sovereignty, not getting up to offer kisses [of greeting] to the duke nor to anyone’." Source: Asbridge, T. "The Crusades: The Authoritative History of the War for the Holy Land", Kindle edition, p. 49

Alexius by this statement clearly express his expectation towards western knights - they need to accept be vassals of Byzantium so all cities, castles or forts they subdue, which were before part of Roman Empire, will be handover to the nobleman appointed by the emperor.

Godfrey, astonished by the splendour of Byzantine court and gifts received from Alexius, made an oath. It was also easy for him as he plans to go back to France after completion of mission - conquer Jerusalem.

Other dukes are not so happy with made such oath, however finally all, convinced probably by Godfrey, did it. 

"The ambitious lesser princes, Tancred of Hauteville and Baldwin of Bologne, each made an immediate crossing of the Bosporus to avoid the oath, but were later persuaded to submit. Raymond, count of Toulouse, alone stubbornly resisted the emperor’s overtures, finally agreeing only to a modified pact which saw him vow not to threaten Alexius’ power or possessions." 
Source: Asbridge, T. "The Crusades: The Authoritative History of the War for the Holy Land", Kindle edition, p. 52

That's briefly how Constantinople episode of First Crusade was described in Thomas Asbridge book. What struck me is how easy Godfrey was convinced by the emperor. Differently and in my opinion more realistic this First Crusade episode was described by Steven Runciman. According to "A History of Crusaders of the Crusades" Godfrey arrive on December 23rd, 1096 and build camp outside city walls. Since than start strength trials between emperor and Godfrey which finally ended just before Eastern of 1097. He finally understands that oath is the only way when his army was defeated on Good Friday by better-trained emperor soldiers. The oath happens on Eastern Sunday, April 5th, 1097.

Finally, crusaders army set off towards Asia Minor - next step will be Nicaea.

Nicaea depicted in Nuremberg Chronicle. 
Source: wikimedia commons

I will keep reading Thomas Asbridge book, comparing with other books and sources, to understand whether that's the only difference or there is more to be found. Feel free to comment and cite other books or sources.

Primary sources:

[1] Asbridge, T. "The Crusades: The Authoritative History of the War for the Holy Land", Kindle edition

[2] Runciman S. (1951) "A History of the Crusades", Volume I - The First Crusade and the Foundation of the Kingdom of Jerusalem, Cambridge University Press

Videos:
[3] The Crusades, Holy War, BBC Two, presented by Thomas Asbridge
[4] THE CRUSADES: CRESCENT & THE CROSS, History Channel 2009
[5] Godfrey de Buillon his life his legend, YouTube RealCrusaderHistory