I've actually been running C&C for a few weeks for an 11, 9, and two 8 year olds (my little siblings) to introduce them to roleplaying. Now, they're bright kids, but that's how simple the game is. It basically has the same 6 stats as all D20 games, no skills, no feats, and set abilities for each class. "Skill" checks and saves are rolled using the modifiers for the stat in question (occasionally adding your level), and you get favored stats that make the DCs lower. It's really simple. The hardest part of the game is picking out gear with starting gold.
How does it compare to 3e, 3.5e and 2e?
I've only played 2e a couple times, but it seems to have similarities, what with the classes getting only select abilities and having extremely well defined roles. As for 3/3.5e, it shares something of a D20 system, but the rules are so much lighter and the classes are more medieval in flavor.
Can I port stuff from those editions to it?
It takes a little work, but it's pretty simple. BAB and Hit Dice basically are the same thing (as in a 4 HD monster has a +4 to hit with C&C). You should probably scale the monster's mean abilities down a hair, or bump it to a higher CR too, because C&C characters and monsters are less powerful than say D&D 3.5 characters and monsters.
What are the core books for it? Would you suggest any additional sourcebooks?
Player's Handbook and Monsters & Treasure. I looked through the Castle Keeper's Guide and found that it really isn't necessary. Those two books have everything you'd ever need to run a game.
What kind of campaign can you run with it?
It lends itself to a low-er magic high fantasy game or a medieval game (if you file out the spellcasters). The Knight class and the Assassin class seem ill suited for dungeons. As a whole, it looks like it would be best suited for a sandbox style game.