Crusader is a solid improvement over the original and the best castle sim ever.
Stronghold Crusader is obviously very similar to the first Stronghold. Not a whole lot has been changed. Instead a whole lot has been improved and refined. It's still a fun city-builder with strong economics, tactics, and maps that usually test your management skills.
The first improvement to the game is the setting. The Medieval times were all about power, and religion was power. The Crusades, while controversial, are a great backdrop for the castle-building gameplay.
The change is more than superficial, but it also has an effect on gameplay. The new desert maps require even more strategy than than the originals forest ans mountains. Farms can only be placed in oasis's and oasis's are rare and poorly placed. In one campaign mission there is a magnificent plateau with only two natural entrances. It's obviously the most defensible location for a castle. Unfortunately, the oasis is rather far away and on flat ground. This means that transferring goods will take a long time, and since you can't build walls everywhere, will be open to attack. This example is perfect illustration of the juggling act you'll play through the whole game. Stronghold Crusader definitely forces you to make sacrifices.
The interface hasn't changed much, but it didn't need to. Some new stuff has been added and things moved around so they make more sense, but the interface was solid and it's solid still.
Graphically, the game still looks nice. The resolution is higher (1024x768) but still quite low. All the charm and detail has remained. Things have understandably been changed to match the feel of the desert. The desert itself looks grand, and the contrast produced by the oasis's is wonderful. Animation has been improved, especially in places like the brewery, etc.
Sound design is still atmospheric. The Arabic music is memorable and definitely a step up in quality from Stronghold 1. The voice-acting is also quite good from sniveling Imams who make your skin crawl to the buff and chivalrous Europeans. It's larger than life, and quite funny.
The combat has some nice new units. The two armies have been differentiated. The Muslims have fast archers based armies, while the Europeans have slow heavily armored armies. The two must be played differently and it adds a nice depth to the game.
There's still quite a bit of replay value here. You get two trails, which are a bunch of linked skirmish maps with rather unique challenges. They;re quite hard and you get three "chicken outs" in case you can't beat them, but be warned they are marked on your record. There's 4 campaigns, each with a good focus of economics and military. The first follows the First Crusade, then the next the rise of Saladin and the fall of Jerusalem, the third on ironically, the Third Crusade, and the last on various other Christian Kingdoms such as the Armenians. Each is well done and unique, and you might learn something.
You still have a free build mode if you want to peacefully build without the pressure of constant attack. There's also multiplayer, and a map editor. I am unaware of any mods.
Crusader is a solid improvement over the original Stronghold. I think it's the best in the series. It's polished and refined almost everything and shifted it to a more interesting period. It's a definite win in my book.