What's the SIEGE Engine? What was it supposed to do, and why did it fail you as a game mechanic?
Hard to sum up briefly, but the SIEGE engine is the "core mechanic" of C&C. It's basically the D20 vs DC, but arrived at differently.
Everything is an ability check, with a difficulty modifier of 1 on up to infinity, roughly corresponding to level--like a spell cast by a 9th level wizard is +9 to the difficulty.
So each character has 2 or 3 Prime Stats, and if he is rolling an ability against a prime stat, his target is 12+the Difficulty (so 21 in the case of that spell), and 18+difficulty otherwise (so 27)
Sounds easy enough so far...except there is no negative difficulty. The absolutely easiest thing to do in the game is a minimum, in regular D&D terms DC 13. So what do the players roll? They roll stat ability (and stats correspond to B/X D&D so they only go up to +3) plus level--if it's a defined class ability, otherwise they just roll with their bonus. So against an even level challenge...and a challenge level of 1 isn't "a challenge appropriate to a 1st level character" but "literally the easiest thing in the world to do" one has at best, a 50% chance of success, even when it's supposed to be your schtick.
Oh, and there's no chart ever explaining what a level 1 challenge is, or a level 5 or 10 challenge--except the guidance that when acting against a creature it's usually equal to their hit dice.
Almost everybody I've ever talked to who plays, including, apparently, the designers, play it as "You either roll against a 12 or 18 except in extraordinary circumstances" which works, but it's explicitly NOT the way the game is designed. I guess I shouldn't let that bug me as much as it does, but I've grown to really prefer games to actually work right out of the box, and C&C has, hands down, the best fantasy art of any game on the market today...by absolutely no exaggeration; I would really love to like the game more than I do.
EDIT: Seriously; the game is beautiful. I have several pieces hanging on my wall. Look up Peter Bradley; he's incredible. It's a big reason reading 5e makes me feel the way Patton Oswalt does eating at KFC.