'An infantry manipule {got into a sack = were surrounded}, and if a cavalry squadron had not cut in from the flank, commanded by me by the way, then for you, philosopher, it would not be possible to converse with Ratslayer.'

If you look at the map where the surrounded troops are marked with a line, you'll see a curve looking like a sack.
Not just a circle but a sack with thin walls that don't let the people break through from inside.

the "by the way" was inspired with the "A" in the original wording, cuz it exerts the same effect on the reader.