River crossings. My first campaign, as the Byzantines, I held the Mongols off on the bridges. I owned Iasi, which I had as a city. The army standing on the bridge consisted of 6 units of Byzantine Infantry, 4 units of Byzantine Guard Archers, 1 trebuchet, 1 general, and 8 assorted horse archers. I lined up 3 units of infantry across the edge of the bridge, with 3 more directly behind them, and all 6 in guard mode. When the battle started, they'd advance directly to the edge of the bridge from the deployment line. The foot archers would line up on a hill where they could fire down onto the bridge. The horse archers would do the same. The treb was set to fire rotting cows, and I'd wait until the Mongols started crossing to begin firing.

I'd always manage to eventually land a dead cow in the middle of the bridge, which meant every Mongol unit would be affected by it. I used the horse archers to chase down any routers that came through my side of the bridge, and didn't pursue any routers across the bridge until the battle was won. In some cases the Mongols would have 2-3 stacks, so there's be a pause after I'd broken the first army but the next hadn't yet arrived, and then I'd switch off the lead infantry.

After the battle was over, I'd use the nearby city and fortress to replenish losses.