Lol, just play it, it's got a simple enough tutorial that should show the whole mechanics. To sort of summarize it...
1. You create a cottage, which houses peasants.
2. You create a whole bunch of cottages and get more peasants. You can upgrade them to get even more peasants, the higher the level, the longer it takes to upgrade and the more expensive it is.
3. You create mines, quarries, woodcutters shops, and iron mines to use those peasants to get resources.
4. Create a barracks and rally point. Then turn your peasants into troops.
5. Create an academy, research stuff like archery and military, and upgrade your barracks. After a while, you get to create pikemen, swordsmen, archers, cavalry, siege weapons. Combat mechanics are rather interesting.. very simple, but gives you a lot of room to experiment. It's sort of like rock-paper-scissors, but you have to sort of get the right balance for it. Archers pretty much trash all infantry, but once you get to them, they get massacred easily.
6. Get your armies to conquer a few valleys, which gives you a resource bonus.
7. After a few battles, you get medals. With enough medals, you get a promotion to Knight.
8. With a promotion, you can build another city. Capture a flat, send a bunch of workers, then build a city.
9. After a while, you amass big armies, build big walls, and can start sending hordes out to conquer your neighbors.
10. You piss off the wrong guy, who calls upon his allies and his allies' allies and they stomp you to oblivion
You don't have to pay for anything, but paying for stuff lets you skip the grind. There's not much bonus to paying, other than you finishing things quicker and not have to sit in front of the computer, waiting for your lvl 7 cottages to be completed.