They also gained wealth through trade. As for Constantinople, historians argue about who betrayed who. The counter argument is that the Byzantines refused them

They also gained wealth through trade. As for Constantinople, historians argue about who betrayed who. The counter argument is that the Byzantines refused them promised supplies leaving them no option but to take them by force or starve. Even the same historian in different books can make it look quite different (Peter Frankopan's description the The Silk Roads reads quite differently from one of his other books).