I always wondered about the way the roman military evolved. I understand the Marian reforms and the early enpire era but it always intrigued me the way the late empire looked.
Opting for the round shields just seems like a step backwards from the effective manipular formations that they had. Is it because these tactics are beconing obsolete? Or is it simply due to cut backs from the declining empire(s)? It really does seem like late empire legions were quite inferior to their earlier counterparts.
You might be interested in some previous threads on Roman weaponry and equipment changes
Why did the Roman shield design change from a squared shield to an oval? - 22 comments, over 1 years old.
The topmost comment directly addresses the issue, though there is some debate on the matter later in the comment thread below it with primary source quotations.
Why did the Scutum fall out of use? - 5 comments, over 4 months old.
This thread addresses whether there actually was a standard shield design at any point during the reign of the Roman Republic/Empire and also discusses the capability to even draw such conclusions with the evidence available.