Another bit about Cannae and roman battlestyles

Romans were taught to fight in formation. i.e. Either the whole line moved forward or noone moved forward. If they did, you can imagine that an individual soldier one step further forward than the rest of his line is a much easier target and can be taken out easily. All roman equipment was designed to fight as a block. Thus the big shields, and short swords. (One would imagine that a smaller round shiel is better for 1to1 fighting).

In Cannae, the romans had no possibility of moving forward, thus expanding the circle as they were surrounded by spearmen with much longer reach and have lost their momentum as someone pointed out before. The celts, with longer swords and charge momentum had also a good advantage as they could charge-retreat-repeat for as long as they wanted.

I would say that the few romans that escaped, managed to do that in the last moments of the battle, as their group would become smaller and smaller everytime, and thus more maniobrable, and they could chose a direction towards which to move, a thing that a 70k men army could not.

My 2cents