Have not played any of Crusader Kings games before this but have some experience with Colonization / Civilization type games. It's basically a conquest type of strategy game where you start around 800AD on a huge map of some territory of Europe + Asia + Africa and play through to around 1400AD. With historically accurate events (more or less), land names, characters and more.

Pick a land, ruler you wish to play as and it starts. From there you find new people to recruit in different positions of your court that will help you run your lands, make alliances through marriage or otherwise, wage wars in name of your house, faith, beliefs and such.

There's very in depth system with land, title, succession claims as well as relationship system with everyone around you. You can find new spouse, befriend someone, find lovers, soulmates or if you annoy someone a lot make a rival enemy. Each character has opinion of you and everyone else and that influences how they act towards you.

Depending on what land you pick it can have various types of faith it follows, with different laws and doctrines, succession, traits and much more. You can also change faith or make your religion with your own laws, marriage rules, doctrines, traits and more. Then turn others to follow your faith which adds more resources you can later use.

Speaking of resources, game has quite few types of those like gold, piety, renown, prestige and other. Each have specific way you can earn and use them to different goals. For example you make gold from your land if you have farms, markets, ports, etc. and can use gold to buy opinion of others, pay for mercenaries, build city improvements and such.

There are also personal dynasty and ruler perks you can unlock from different primary "stats" trees your ruler has, like learning, stewardship, marital, intrigue and others. Those stats are randomly given to people at birth depending on their parent stats. There are also many positive and negative traits you can get throughout your life on your ruler that add variety of interesting effects. Everyone ages in game so you will have succession and depending on laws and claims the lands can often change ownership but even if your ruler dies you can always continue playing as long as you have children and heirs.

Game has bunch of random events you can either take part yourself or randomly trigger depending on people and their relationships, opinions. Like someone declares war, you go on pilgrimage; marry someone; scheme someone; make a friend or lover; have a kid behind spouses back with another ruler; go on hunt; have a feast; someone gets wounded or sick and much more. All of those have many choices you can pick with different outcomes or success rates. Majority have interesting text accompanied that tell little story too.

I could have written a lot more about each topic and other aspects of the game but will leave it there since game has A LOT of depth and different simulation systems.

Optimization is good, game runs pretty good even with all that is going on. There's multiplayer so you can game with friends. Game doesn't take much space (around 5-10gb) or use whole lot of hardware. With 50 hours of gameplay I only experienced one crash. It's fun game if you like development of countries through different eras, micro manage people and armies and numerous events all over your lands.