The Siege Engine is used in place of a skill system. You choose 2 stats as your "primes" (IIRC, 3 for humans). All checks are stat checks and the base DC is 18 (I think), but 12 instead if the stat you are using is a "prime" stat.
Seige is a big part of what makes the game different. A DC of 18 for any skill not associated with your primes is a hefty obstacle and at lower levels even the 12 DC can make you sweat a little. If you follow the rules as written, it certainly ensures a good deal of niche protection. It also means that you oughtta be running the game in an older-school style, where not every activity requires a roll.
It's well made, and is indeed D&D 3.x made simple. However I point to Gruff's link to Erin Smale's house rules and note that a good deal of them are rules from the Seige Engine tweaked to fit in RC D&D and makes something genuinely old-school in flavor.