Crusader Kings 2 is a complex grand strategy game. The most complicated game I've ever played. Be prepared to spend hours and I mean hours confused as hell, looking at youtube, the wiki and forums. Once it clicks though, it's awesome. Not for everyone, but its kinda like playing through 'real life' Game of Thrones (there's a game of thrones mod too).
You control a land owning character starting on any date between 1066 and 1453 AD (The Old Gods DLC will push this starting date back to 867 if you wish). You can start as a powerful emperor or a lowly count. The map you play on is more or less Europe stretching into parts of Asia and Africa. Your sole goal in the game is to keep your bloodline going (and I believe holding land). Other than that, there isn't a win screen, there aren't defined goals, its open ended, you play until 1453AD and then the game ends (save exportable to Europa Universalis 4 to continue your game with the purchase of a DLC). You get a 'score', but that is just a rough estimate on how prestigious and pious your bloodline was. It's quite possible to have 400 years of super rewarding gameplay without getting a high score. This is a trend you will see me developing on and is a big part of what makes CKII so awesome.
That's kinda hard to grasp, but it is less awkward than it sounds. The goals sort of present themselves naturally. You don't often sit there with no idea what to do (again, once you understand the game), you kind of always have something you're working towards. For a first time player this is usually just become as powerful as possible, i.e. aquire land, become a king, then become an emperor and expand your empire. If you're an advanced player you can set your own crazy goals, roleplay, or do other things to spice up the game.
The game runs in 'real time', with a day going by a few times a minute on normal speed. You can fast forward and pause. Most of the time you will indeed be going through various menus. Arranging marriages, keeping tabs on powerful families, plotting assinations, acquiring territory through political shennigans, keeping your subjects happy, bringing in powerful characters to tutor the children of the realm and hold office, and much more. You can micromanage to your hearts content, play a more hands off style, or switch between the two.
When combat breaks out, it is done by moving stacks of armys around the campaign map, meeting an opposing army will cause a battle. These really just compose of watching numbers go down, and kills go up, so if you're looking for Total War style combat you arent going to get it here. Don't be fooled though, many times I've been on the edge of my seat watching these numbers change. The combat system is quiet nuanced as well. To be the most effective you have many things to consider. Terrain bonuses, attrition penalties, appointing competant commanders from your realm, and more add depth.
In all I highly recommend this game to anyone who has played something like Civ or Total War and fancy themself a strategy gamer, is looking for a challenege, and has a lot of patience. This is kinda the next step you can take into something quite a bit more complicated. That is the path I took into the game am loving it.
The game is still actively having expansions and patches developed for it as of the writing of this review 11/25/2013.
Full disclosure: Your reviewer has not quite finished a playthrough yet but am close. I have 120 hours playtime. I own the dlcs The Old Gods, Legacy of Rome, and the Europa Universalis 4 save converter. I solely purchased EU4 because of how much I enjoyed my crusader kings 2 game and wanted to continue playing my empire, I haven't even played it yet.
Also: Don't buy for full price. Even though it is worth it, getting the game and all the dlc all at full price is pretty expensive. The base game is often on sale or in bundles.
This is my first steam review so if you think this is helpful give the thumbs up and I'll do some more reviews. Hope you enjoyed reading.