As mentioned elsewhere, this reads almost like fiction ... except it's real, painstakingly sifted and pieced together from a multitude of sources on both sides. The picture that emerges is more complex than I expected; it's about far more than Sultan Mehmet turning up with his huge army and battering the walls down with his great siege guns. Just as important to the outcome was the machinations going on behind those walls and in Christendom as a whole; this is a story of divine portents and tragic schism; of Christians taken and converted (or not) to fight their erstwhile brethren; of commercial greed and rivalry that sometimes took precedence over shared faith, culture, and strategic interests.
Most poignantly, it's the story of a doomed emperor standing with his allies and subjects against overwhelming odds, determined not to be the one to surrender a heritage of 1000+ years and the last living link to antiquity.
The author brings out several turning points where things could have gone differently, that make you wonder "what if?" ... but even as you do so, you realise -- because of the broader picture that he paints -- that even if Constantinople had survived this particular siege (as it had so many before) its ultimate fall was inevitable.