From publisher blurb:
CASTLE OLDSKULL (“Old School”) is a well-regarded, system neutral line of supplements designed for use in Fantasy Role-Playing Games (FRPGs).
Ideas are presented in such a way that they can be used or customized for any edition game, from the 1970s to the present day. These volumes exemplify the iconic “sandbox,” do-it-yourself, and free-form ideals established by the original Lake Geneva campaigns, c. 1972-1979. Respected sources of inspiration include Arneson, Barker, Bledsaw, Burroughs, Dunsany, Gygax, Holmes, Howard, Kask, Kuntz, Leiber, Lovecraft, Merritt, Moldvay, Moorcock, Sutherland, Tolkien, Trampier, Vance and Ward.