This game actually emphasises strategy. Unlike other RTS's which whoever clicks the fastest or spawns a rush with low-level units in the first 2 minutes of the game wins, Stronghold instead requires planning.
You need to determine how big to build your castle and whether to place building inside or outside the walls. Too small a castle and a long siege will leave you starving and destroy your industries. Too large a castle and your opponent won't need to breech any walls because you never complete them.
If defending a castle sounds too easy, don't worry because then you will need to lay siege to your enemies castle as well.
Your success usually depends on keeping your peasants well fed and happy. If you run out of food or force them to pay to much in taxes and they will become unhappy and leave. Once the start leaving, they stop working in your industries creating a cascade effect. It is very difficult to recover from this type economic nightmare.
Also, the game does add some humorous elements as well. If you play a long time or late into the night the game will suggest that you take a break or go get a snack.
If you have never played stronghold do yourself a favor and buy both it and its sequel (Stronghold Crusader) now. The only problem you will have is the lack of sleep why you try to finish one more level.