Crusader Kings, and in a couple years, Crusader Kings 2 are probably the most likely games to try out if you're into this sort of thing. It does fit the description in the OP, but is much more at the same time. You are not merely concerned with expanding your own territory, and your own wealth and power; ultimately you must think about securing a future for your children. While it is certainly tempting to go on vast wars of expansion to make yourself wealthy and strong, the second your son succeeds to your throne, you are almost guaranteed to have it all fall apart since he is inexperienced, and doesn't have the loyalty of all the men who were loyal to you. In addition, other powers who are watching you closely for a chance to retake territory you took from them will look at your son's realm hungrily.
So yeah, in the beginning, you do just feel like a thug defending your territory, and enforcing your claims to the land in order to support yourself, but as your character grows older, close to death, the game takes on a very different feel. Then, when the son is in power, the cycle repeats itself.
Beyond that, I've wanted for a while to make a better simulation with the same sort of feel, but lack the programming finesse to do it myself.
On the other hand, I feel that the description provided by Mainiac is severely lacking. Really, Feudalism as it is understood today never really existed. The idea is a purely modern convention that vaguely describes post-Rome dark ages Europe in a very general, half-assed sort of way. For every example that you will find that mirrors the idea of feudalism closely, you will find another that turns it on it's head. Just look at the Medieval lowland countries, or the Russian countries, or Poland, or Italy, or even the East Roman Empire.
More than anything, my view of Feudalism through the dark ages and middle ages is that it was more about maintaining the legitimacy of rulers more than expanding personal power. A ruler who would arbitrarily go to war for no reason other than to expand his personal power is not respected by his people, for he's merely throwing lives away for his own sake, or obviously his neighbors. The wars that occurred were not petty power grabs in the sense that a person would arbitrarily declare they want a piece of land, and take it. Rather, they were mainly sparked because two rulers claimed to have legitimate right to rule over a given piece of land, and seek to enforce their claims. Just examine the causes of the Hundred Years War to get a grasp of it.