For the battles I've done, it's been around castles. Defending a castle against approximately Company-sized (150 characters) attackers can work well for a party plus perhaps an understrength platoon (20 or so) grunts.
I did that once with 2nd level PC's in the Keep on the Borderlands against an army of undead, bandits, and some low level PC types. The PC's fought this mano-a-mano. I gave them lots of potions to keep healing up. It was the best and longest fight (153 rds of 1st Edition play) we ever did.
I also did it with 4th-5th level PC's in a small tower complex against an army of mostly human 1st level warrior types, with some orc sappers (combat engineers), giants as artillery, and spell casters. This involved more PC's taking out the leaders. The critical hits from archer formations did become a daunting issue, so the PC's had to take total cover from that.
And I did a PC-party only (4th-5th) level assault on a small motte-and-bailey castle (with bad guards and damaged walls) with a humanoid reinforced platoon-sized force (60 or so) with a tank (war rhino), air support (tauric hobgoblin griffon), and some humanoids with levels. This fight was a bring-it-on grudge match. Very fun. I based it on the Dzeebagd module, but heavily modified it.
Using a castle makes things simpler -- more contained map, more contained fighting where the PC's can excel, and just fun!