When combat involves characters who are mounted (e.g. on horseback), there are a number of modifications to the rules.
Mounted characters take up more space on the combat board than characters on foot. Mounts also move differently from characters on foot and can also attack opponents. A mount may become agitated or rear in combat, which affects his/her rider's ability to stay on and control the mount.
A mounted character's combat speed, and ability to attack, block/parry and dodge are all restricted by his/her Riding skill; hence, a good rider can fight as well as he/she does on foot (or even better in melee, since a mounted character's height advantage makes it easier to attack and he/she may also get a charging bonus on damage), but a poor rider will find it difficult to fight while mounted.
A mounted character can be knocked off his/her mount during combat, and may suffer falling damage as a result. A character on foot armed with a spear or pike can brace his/her polearm to increase his/her effectiveness against a mounted opponent charging him/her.
A mounted character striking a target on foot or a character on foot striking a mounted target has different modifiers for aiming attacks at areas of the target's body than normal.