Well the MP is pretty basic, you have Deathmatch, Team Deathmatch and the Battle mode where you only have one life per round, Invasion is similar but you have to assault a castle. You can choose your troop (cavalry, infantry, archer/crossbow) and for kills you get money with which you can buy better weapons and armor. Basically you only need one kill and you can buy all the good stuff.
It is not the most elaborate MP in the world but it is good fun. I prefer the Battle mode because people play more careful here. After a week of playing it I finally manage to get some kills with the crossbow. The large 100 player servers are awesome but they tend to crash and you shouldn't get killed too early or you have to sit out the rest of the round. Considering that there is no other medieval action game out there (if you are interested in it) and the small but dedicated community it is worth to keep your eyes open for it (when it is on discount).
SP is a bit tedious in my eyes will play that when I have more time and patience.