Those systems use a d20 based difficulty check system. Basically the task at hand is given a Challenge Rating (CR) by the DM which the player then rolls a d20 for, adding or subtracting the modifier for a specific skill.
The SIEGE engine on the other hand is used in games such as Castles & Crusades.
Improvising is great! I still need a lot of work on that.
RE: Check Your Abilities