The entire Total War series (Except for Viking Invasion, never got it). Modded Rome for months because vanilla was basically unplayable to me, and when I was done I lost interest.
Age of Empires I and II (II is nothing fabulous but it was fun for a while)
RPGs like Morrowind, Baldur's Gate I and II (altough II somehow dissapointed me), Icewind Dale I and II (IMHO better than BGII, at least it admits it just a hack&slash
), waaaay off topic but speaking of RPGs Fallout I and II: IMHO by far the BEST RPG EVER made. By the large margin. Oh and Diablo II. It's not even a RPG in my book and I actually hated it for it's linearity and go-and-kill-them-all only quests, but it was always "just one more waypoint before I go to bed" and so until the dawn.
Something is scary in that game's addiction. Never played Diablo I.
Almost forgot. Civilization series. Not actually medieval only, but related to the topic. Original Civilization is one of the two best games I ever saw. When I got it first, I didn't speak one word of english so figuring things out was an adventure for itself.
Another is Centurion. Not medieval, but close enough. Very similar in concept to Total War games (turn based strategical part with provinces as fields and real time battles), just on the 386 level. Those were the days...