The ideal legion in a single player game is quite simple; you'll want to have comprised mainly of heavy infantry, heavy cavalry, and horse archers (if you can't get horse archers just use cavalry).

Most terrain is restricted to a certain combat width, with the lowest being 16 and the highest being 40. With that in mind you'll want to structure your legion around this. Another important unit are engineers, you'll want at least 10% of your legion to be engineers, because if you have one engineer cohort for every 10 other cohorts in your army you will take no river crossing penalties. With this in mind the size of our legions now have a minimum size, and a maximum size.


Now, for our composition we need to keep one more thing in mind; there are three organized parts to your legion, flanking units, primary units, and secondary units. Flanking units will always be to the far left, and right of the battle, they use their high flanking range (mobility) to deal damage to other units, this is where we'll put our cavalry archers. The flank can also have a set size, which determines how many cohorts will be there (max), with this in mind it's a good idea to minimize the size of your flanks to 5, size 5 means that up to ten cohorts will be flanking at a time. Primary units are always forced to the middle of the battle no matter what, here we put our heavy cavalry we'll want 16 cohorts of heavy cavalry, because they are expensive, and past that they become obsolete. Secondary cohorts are forced as close to the middle as possible (behind the primary cohorts), therefore these units are our canon fodder, they will fill the line, and reinforce any unit that falls.

With all that said we can make the ideal legion, 16 cohorts of heavy cavalry as primary position,6 cohorts of horse archers* as flankers (size 5), then we'll want 18 cohorts of heavy infantry as secondary units, finally we'll want 5 units of engineers, and 5 units of supply trains; this legion will be 25 thousand units strong and ideal for the maximum terrain defense, but also operational for other terrain defenses.

*You only want 6 Regiments of horse archers so that you force the game to put only 3 units in each Flank.