I've ranted before about how I'm constantly purging my gaming books. If any books/system sits on my shelf for any long period of time I get rid of them.
Today I was looking over my C&C books and something dawned on me. Although I haven't run C&C in a while I've been using the books constantly.
When I ran my Swords & wizardry game. It was a C&C adventure I ran.
When I ran basic fantasy game I used the codex books. When I ran my last 3e game I borrowed heavily from many C&C book and used bluffside as the setting. Even now in my 5E game I using many of my C&C books.
Without even mentioning the core mechanic of the system (Siege engine) Castles & crusades is a d20 system designed using the srd to replicate the feel of older editions of D&D. This makes it a middle ground system. Compatible with very little work with old school editions and its clones. Compatible with most everything with material from the d20 era games. And even close enough to draw inspiration and resources for use with 5E.
As for the core mechanics that make up the siege engine. They are modular enough to change to taste. The castle keepers guide has a lot of options for changing the siege engine to the system that works best for your style and preference. Most of the core rules of 5E were already in use in C&C. Other 5E mechanics like inspiration, advantage, and disadvantage could all be used in C&C with little to no changes. Flexability through being modular.
I might like a lot of different games. I run a lot of other games. But it seems C&C has become a major tool in my tool box. Not just another game.