| bigmac |
The answer to what system the setting uses: OGL
OGL isn't a game system. A lot of game publishers like to imply that OGL means d20 System, but all it means is that the product contains a copy of the Open Game Licence.
There are plenty of products that use the OGL, but are not compatible with the d20 System. If you want to insert a product into a D&D game, you need to be careful that the product isn't one of those.
(I'd rather they had called this "Ed Greenwood's Castlemourn Campaign (SRD) Hardcover". At least with the SRD, you know that you are getting something compatible with D&D.
I can confirm that this RPG is compatible with the d20 System. But if MWP ever renew their licence with Ed Greenwood, they are almost certainly going to want to switch any new products over to their Cortex System. So if you buy this watch out for that happening.