This game was from way back when they included DOS and windows versions together on a CD Rom. If you do want to get this game (there is a Lords of the Realm Royal Edition on gog.com which includes the original, the second and the expansion for the second, siege pack for 5.99) I would suggest a digital copy, there is a test to see if your computer can run it, if your computer is too good (anything windows XP or better is probably too good) an error message will pop up saying something like "Interger divide by zero" and it will NOT let you install the game again, there is a work around, but the problem I had then was it would not read the CD to play the game. I've had to play the DOS version on my old 98 PC because of this and it's kind of annoying.
That said, this game is a basic medieval strategy game, the king is dead and you are pitted against between 1 and 4 other lords (the knight, baron, bishop and countess) to fight for the Kingdom. You need to balance military conquest with maintaining your population (hapiness, food supply, etc.).
I would say that in some ways I can compare this game to Civilization, but only in the way that it is a strategy game that is simple enough to pull you in but complex enough to keep you there for days and days on end, and whether you win or lose, you had fun. Sometimes if you lose over and over again you go away sore.
I cannot stress about how important it is to maintain a balance in this game, if you have no military but the people are doing great and love you, you'll get floored, unlike Civilization the only way to win is through military conquest. On the other hand, if you keep raising armies and taxes to fund them, your people will get pissed off and either revolt, in which case you lose the land, or they will emmigrate to another county (this being said, pissy neighbors can be good for you!)
The game does have a basic diplomacy system, you can send a gift of crowns (money), send a compiment, send an insult, offer an alliance, and (if you are in an alliance) ask your ally to help you. You do need to maintain relations with your ally, however, personally I never feel like giving them money just to keep allied and my alliances always fall apart, that can be good in a way too because it keeps one lord off your back for a bit.
Combat in this game I've never really been a huge fan of, it's your basic combat, your units on the field VS. enemies units, one single troop can represent multiple units though, and each time one unit dies, that troops..."HP bar" goes down by 1, thus making large scale battles into small scale ones that old computers could handle. The game does give you an option to autocalc battles, usually your side does worse though than if you actually fight out the battle.
The siege pack expansion essentially adds skirmish battles, you can either do a regular skirmish, a siege attack or a siege defense (it also includes a basic map editor) with options as to army size. The major bug in Siege Pack is that it runs on points, since peasants are the lowest point costing unit you can have 900 peasants to your enemies 300 trained troops and win every time (maybe that's what made me dislike the combat?).
Overall this game is an excellent strategy game, I've had freinds complain they could never find the right balance, they always got smashed or their counties always revolted, I've only had this problem on high difficulty games, so maybe I'm just randomly good at this game, who knows.