@Charles Lowry
Had a couple more thoughts before heading for bed. So very sorry for the spam. Can you tell C&C gets me excited?
One, I play Prime/Non-Prime attributes' Challenge Base as a
strong suggestion, but if makes sense at the time, I don't hesitate to treat a Prime check as Non-Prime or a Non-Prime checks as Prime. Very similar to the situations in 5E where you would reward Adv/Dis!
Think of Fighter who has a background in medicine trying to recall what plant relieves hangovers. Even his INT is Non-Prime, let them roll with a CB of 12 anyways!
Second, SIEGE checks seem dead simple at first, but the more you play it, the more you realize how powerful of a tool it can be in the hands of a creative DM.
Take the Ranger's use of traps. It allows the Ranger to make a Wisdom check to set snares and pits.
What if the Ranger wants to set up a more complicated trap, like something to trap an Ogre?
You could just say no since it isn't in the book. More interesting though is to rule all simple traps are CL 0; more complicated ones higher. For instance in this case you could rule CL 3 (Ogre's HD).
What if a Non-Ranger wants to set a simple trap?
Again, you could rule "no" since it isn't in the book, but what if a situation comes up where it would be a lot of fun to try? You could houserule on the fly that when a Ranger sets traps, the CL for the creature to avoid the trap is equal to the Ranger's Level; anyone else would always just set a CL 0 trap.
Further, you could rule that if the Ranger is successful on his check, the creature tripping the trap doesn't get a Save. A "failure" to the set up the trap still produces one, but it affords the creature a chance to Save (CL 0 maybe?).
Just too cool.