They stopped using the phalanx because they found that the maniple gave them more mobility on the battlefield.
Chat with our AI personalities
They used several different battle formations instead of the phalanx. The Roman maniples and cohorts were flexible as opposed to the relatively static tactics that had to be used with the phalanx. This manoeuvrability gave the Romans a definite advantage over their opponents.
The Romans abandoned the phalanx because they found that the mobility of the maniple was much more effective. The phalanx was a close formed, rigid formation, whereas the maniple (and later cohort) could be deployed in various ways to better suit battle conditions.
The Romans abandoned the wall by about 410AD, and withdrew from England. There were various reasons, among which were; Increased Barbarian invasions, military coups and econimic decline.
What advantages did the Romans find in using a cross vault instead of a barrel vault?
It looks like a square shape.