Despite the same engine and on the surface, similar mechanics Stronghold: Crusader plays very differently in single player to the original. Almost always either the attacker or the defender, in Crusader you are always both, needing to both defend your own castle and assult your enemies. Because of this the concept of a siege really comes into its own - it's difficult to build a castle with all your resources inside the walls, thus it while taken on an enemy fortress at full strength can be near impossible you can take the middle ground and deny access to rare farmland and stone/iron and slowly wear an enemy down.
The ability to use fire as an offensive weapon and the additional strategic opportunities afforded by the arabic units are fantastic additions. There are some flaws and balance issues, it's too easy to rebuild your castle while under assault at the market makes it too easy to do without some resources but overall it's a fantastically replayable game.