This is a dumbed-down version of the excellent system by Vic Lineal, found from the Fanaticus website. A link to the original is below.
In Spring 1231 The Mongols attacked into Kaegyong and a standard 12 v 12 battle was fought. The terrain turned out all to be packed into two quarters, and accordingly the Mongol commander (me) chose that side. The idea was that if the Koreans had all those steep hills to occupy the Mongols had nothing to winkle them out with. In terms of real-life events the Mongol scouts knew that the Koreans were moving to block the passes out of the hills and hurried to catch them exposed on the plains.
That was the last thing that went according to Mongol plans. I'd not allowed for the Koreans deploying as far forward as they could, so there was very little room for the Mongols to expand once they got out between the hills, and that area was all within bowshot of the Korean archers who had by then advanced to form wings which could shoot, Agincourt herce style, into the Mongols. Some Mongol light horse got out of the area and attacked the Korean left, but the guerrilla pikes there held them off after a bit of a struggle. In the centre the Mongol heavy cavalry tried to get out and charge the archers, but before they could form up the bowmen had shot down three elements of light horse and the guerrillas accounted for another, and that was the game over.
In the following campaign turns the Mongols recuperated, gathering reinforcements, and the Koreans did not advance into Mongol-held territory. This was a bit puzzling, but the Korean commander (Mr G- P-) later revealed that he was hoping for a 'two repelled invasion' win.
Once they were back at full strength the Mongols invaded again. The terrain was more evenly spread this time, and the Mongols feinted an attack on their right, where the Koreans hurriedly occupied a difficult hill with some archers, then switched to an attack on their left, where the Korean cavalry guarded an open flank. This initially went well, and the Korean horse-archers were overwhelmed by Mongol light horse. More light horse moving to reinforce there got embroiled in a confused scrap initially involving the Korean heavy cavalry including the general but escalating to involve odd elements of spearmen detached from the main line. The Korean bowmen on the Korean left were a mixed blessing for the defenders: they effectively blocked the route from the Mongol right across the front to help out on the left, as it was all within range of two or three elements of archers, but they were also impossible for the Koreans to move as being on a difficult hill their command radius was reduced and the general was busy away on the Korean right dealing with the outflanking Mongol light horse.
Things seemed to be going well for the Mongols when a single element of light horse went wide of the hill and its archers and took the Korean camp. It also looked positive for the invaders when they managed to gang up on the Korean cavalry and push them back. However Mongol light horse transferring from their right to reinforce the success on the left were obliged to gamble on passing through the valley of death in front of the massed Korean archers, and a couple of elements didn't make it. The rest of these light horse went on to skirmish inconclusively with the spearmen in the Korean centre. After a few close escapes the Korean general managed to 'close the door' and hit some Mongol light horse in the flank, and when another was pushed off the table by the other Korean cavalry that was it.
After two repelled invasions the Mongols wrote off Korea as too tough a nut to crack just now and went off to commit massacres in the Middle East instead. The general was transferred to an honourable but extremely dangerous post leading assaults on Russian cities.
Unfortunately for Korea the Mongols will be back - possibly as Yuan once Kubilai comes to the throne.
As a sort of a postscript we tried a game where the Koreans faced Yuan. The wide variety of types in the Yuan army gave me lots of options but in pre-game planning also caused 'analysis paralysis'. All my plans went in the bin anyway when the Koreans turned out to be the attackers.
Above are the initial dispositions. The Yuan have archers (2 x 4Bw, 1 x 3Bw) and artillery (1 x Art) in the centre, with halberdiers and swordsmen (1 x 4Bd, 1 x 3Bd) directly behind the archers with a vague idea that the bows can recoil through the blades who can then attack whoever drove the archers back. On their right is the heavy cavalry (2 x Cv), on their right the light cavalry (3 x LH) and the general (Cv) is in reserve.
The Koreans have their archers (4 x 4Bw) on their right, opposite the Mongol light horse and artillery, supported by their own horse archers (1 x LH). Their spears (4 x Sp) make up their centre and left, with the guerrilla pikemen (1 x 3Pk) and heavy cavalry (2 x Cv) behind the left.
Above is a view of each side from behind the other side's lines.
Above: The Yuan artillery is carried into range of the Korean lines and set up. In a couple of turns it destroys two elements of Korean archers. The rest, and the horse archers, retire out of range allowing the Mongol light cavalry to move round and threaten them. Not looking good for the Koreans.
Above: With their right in disarray the Koreans advance boldly in the centre and on the left. The Yuan swordsmen and halberdiers expand to meet them and both sides' heavy cavalry and generals square up on the Yuan right.
Above: On their right the Koreans reorganise, with 2 x 4Bw and 1 x LH preparing to take on the Mongol light cavalry. They assemble out of range or arc of the artillery. In the centre the Yuan archers run out of targets; Korean spearmen clash with Chinese swordsmen and halberdiers, and both the generals lead their heavy cavalry into combat.
Above: The Korean spearmen destroy 1 x 4Bw and the 3Bd, leaving a gap on the Yuan centre-right. Some light horse come over to plug the gap, and the Korean 2 x 4Bw remaining on their right deal with the Mongol LH left there.
Above: The decisive moment, as with the heavy cavalry locked in a prolonged melee the peasant pikemen get involved and the Yuan general is killed.