Citadel's game and setting are licensed under a Creative Commons Attribution-Share Alike license. This license lets you remix, tweak, and build upon this work even for commercial reasons, as long as you credit and license all new creations under identical terms.
This game is a spiritual follower to Breaking Worlds, the previous CLS system, and took a large proportion of the rules from it.
Citadel rules include some definitions adapted from Profound Decisions' LRP system, Empire.
Various rules explanations have been carried over from the former Cambridge Treasure Trap system.