Maybe not so original but three units praetorian cohorts, three archer auxilia, and four onager units to hold a fort against a full stack of gauls, noble cav, swordsman and chosen swordsmen with some missile units thrown in for good measure. I put the cohorts exactly on the edge of the square facing the gate, the archers behind them and the onagers at the very rear of the fort. The praetorians jam up the middle, the gaulic hord just kind of
throbs there, and the onagers fairly quickly thin their ranks.

In the campaign game I'll throw a few of these forts up in an enemy territory I want to take in the next few turns. The enemy grinds its full stacks against the forts (which sometimes get taken) and when my main invasion force arrives its pretty easy going.