With a steep learning curve it can be hard to get into, but once you're in theres no getting out!
The game can be very frustrating at times. For example when you start off you'll probably not have enough food to feed your people, then the people will be unhappy, no work will get done, no soldiers will be recruited, and you'll struggle to stay in the game. You could also screw yourself over in the beginning by building many buildings and then if you have no woodcutters and not enough wood to build a market, your economy will be severely crippled and you'll certainly have to restart. As you get more experienced, however you'll become more proficient at taking care of your people and you'll be able to focus more on combat. This is when the game starts getting fun. At first simply creating a large army can win you the game, but when the missions get harder that simply won't cut it.
The missions get harder in several ways. The number of enemies is increased, the amount of nearby resources is reduced, certain resources may be eliminated, disease or fire could become an issue, the enemies become more difficult or wild animals could attack. The shear amount of possibilities in the game make it extremely addictive. The player must juggle his/her responsibilities of keeping the people happy, building an inpenetrable defense, building a stable supply of income to finally fund his/her army.
Attacking a castle can be a daunting task, you could have to overcome walls, moats, traps, towers full of enemies firing stones or missiles, and finally you win by killing the enemy lord himself who is a warrior to be reckoned with. This is hard enough in single player, but in multiplayer it can be even more challenging. There are also an abundance of seige weapons that can be instrumental in successfully attacking or defending a fortress.
The game has a perfect balance between complexity and simplicity, you simply will not find a better game for 5$ and you'd probably have to spend over 50$ to come anywhere close.