It is funny that I picked up this book a couple of weeks after I had finished reading a book outlining the Roman Republic, and what was even stranger was all along I thought it was a Biggles book (not as I was reading it mind you). As it turned out, it effectively continued on from where the previous book on the Roman Republic had finished (not that this the two books were a part of a series, rather it just happened that when I bought them they ended up being complimentary).
I am actually inclined to think that the first three centuries of the Roman empire is probably the best part to look at because it was during this period that the empire was at its height, though one could suggest that the barbarian invasions between 240 and 270 AD was effectively the beginning of the end. However, after the barbarians were eventually defeated, and peace was restored to the empire, the empire still had a couple of centuries to run before the complete collapse of the West and the gradual decline of the East.
However there were some significant changes after the barbarian invasions that suggested that classical Rome, and in particular the classical world, was coming to an end. Robin Lane Fox, in The Classical World: An Epic History from Homer to Hadrian believes that the end of the classic period was the Emperor Hadrian, and in a way he is probably correct because after Hardrian there was nothing really all that new coming out of the empire in the way of art, architecture, science, or even literature. Mind you, on the literary front, we can't forget the huge amount of coming from the Christians (much of which is preserved) but when it comes to the pagan authors there is actually very little. It seemed that as we enter the 2nd and 3rd centuries, people are too busy looking back at what had gone and not too concerned with looking forward. Also, they were in a time of unprecedented peace, that was pierced by only one year of civil war, and without conflict people ended up becoming fat on their indulgences. In many cases it is much like the United States of today, where the heartland is relatively free from enemy invasion, and has been for over two hundred years.
As for the end of the third century, the empire had changed dramatically. The administration of the empire had moved out of Rome, and with Constantine's victory over his rivals, the capital was moved east to the city of Byzantium (to be renamed Constantinople, and later Istanbul), and Rome simply became a symbol of a bygone age. Further, Constantine had allowed Christians to openly worship, and was also said to be Christian himself (though that is still debatable, and my position is that a monotheistic religion made it better for stability than a polytheistic religion, and I will leave my criticisms of Eusebius for a later time). With the acceptance of Christianity within the empire, and with it soon becoming the state religion, the old empire was gone forever (though there were the occasional apostate emperors who ascended the throne, but generally, after Constantine, most of them were Christian).
As for this book, I got the distinct impression, at least at the end, that the author was Christian. Okay, I certainly agree with his position that we cannot criticise Rome using twenty-first century values, but I also noticed that by the end of the book he seemed to be cheerleading Christianity and praising Constantine's reforms, to the point that he was arguing in favour of the first Christian emperor. Mind you, he was also very much cheerleading Rome, but then again, I tend to do the same thing when it comes to Classical Athens.