It was probably luck more than anything else, but in an old Rome 2 Egypt campaign, I managed to take a Seleucid city (Antioch) with ~2500 men that was defended by ~4500.
My force wasn't anything special, about 4 pikemen, a Royal Thorax Sword general, some Egyptian infantry, 2 cavalry, 4 archers, and 2 Giant Greek Ballista. The enemy had a 14-unit garrison of mixed troops, plus a full stack of reinforcements for the battle that I hadn't seen beforehand. I decided to give it a shot in manual battle anyway, and set up my normal siege formation; ballistae aimed towards the wall at an area without a lot of nearby towers, focused on blow open a hole for entry and taking out some of the defense towers so that we didn't get beat up on the approach. The infantry and ranged troops were set up to defend the artillery from any counter-attacks, while my cav was kept a bit towards the back on the flanks as a guard.
As soon as the battle starts, I suddenly realize that the enemy reinforcing stack wasn't set up in the city already as I had thought. Apparently, they were at sea within range of the city, and were now headed up towards the beach right near my artillery position. Seeing a golden opportunity, almost all of my melee and ranged infantry got moved to defend the beach they were rowing towards, while my ballistae focused on making a hole. The garrison seemed interested in trying to keep the city defended and didn't both to launch any sort of strike on the artillery, letting me focus on the troops trying to land.
Half of the full enemy stack came ashore in one place. They took a bunch of casualties from the archers while trying to land, and once they made it to shore, they ran full-force into a row of pikemen that poked them from inside a phalanx until they all routed. The rest of them went for a secondary landing site, but it was mostly low-tier spear infantry who ran straight into a waiting force of Egyptian infantry led by my general. By about seven minutes into the battle, the entire reinforcing army was in full retreat, with relatively light casualties on my side (I'd lost maybe a tenth of my force, and that was mainly from missile troops who'd bombarded the defending melee infantry before landing).
All this time I'd left the ballistae on Fire at Will, and even though they were now out of ammo, they'd blow a decent sized hole in the wall and caused a fair number of casualties to the garrison troops who'd tried to hang out on the wall as defenders. After a few minutes idling so that the troops could rest, the melee infantry got sent in to clean up the city, with the archers providing ranged support. Most of the garrison routed quickly after how badly their reinforcements had gotten whooped, and by the time I pushed all the way to the capture point and took the city, I'd lost only about 350-400 troops. The enemy ended up losing the better part of a full stack on top of the city getting conquered. It seemed to cripple their attempts at a counter-offensive as well; the last Seleciud city along the Mediterranean coast was nearby Tarsus, which full a few turns later to the same army that had taken Antioch.
TL;DR got lucky and enemy was stuck on boats, poked boats to death with archers and pikes.