The rules are familar enough to anybody who likes RuneQuest style games, although there are a few differences in stats and skills. The setting is reminiscent of Ars Magica in it's 3rd edition - being a very dark, medieval world with a very high level of authenticity based on historical research.
Set specifically in the Spanish Peninsular (this was Spain's No.1 RPG - translated for the first time into English) during the 13/14th Century (High Medieval Period), it's less structured than Ars Magica and there is no default campaign as such. However, the emphasis on demonology, based on the Christian, Jewish and Islamic faiths of the time, as well as the name ('Aquelarre' means 'Coven') suggests that the characters will possibly be dabbling in the dark arts of magic or at least dealing with them. Magic is based upon alchemical lore, again well researched, evocative and with a big spell/ritual list, and there is a comprehensive host of angels and demons involved in the background, as well as a bestiary featuring creatures from Spanish folklore. There is a clever core mechanic where characters are rated on a two-headed scale of Rationality/Irrationality. This serves as a Sanity mechanic, of sorts: The more Irrational you are the more you believe in the supernatural (and the more capable you are to practice magic), but the more Rational you are, the more resistant you are to it.
The time period has the Spanish Peninsular made up of five competing nations, again well researched and presented here with lots of information about the history and medieval society. Supplements will detail each nation seperately - but there is lots here already. Nobody expects the Spanish Inquisition (!) but you could well play in a campaign based around this sort of investigative organisation (or have them as antagonists), along with a lot of other societies you could join (including military orders or fraternities). There are several introductory scenarios included.
This is a very rich setting, beautifully illustrated in a massive book, which is very much a work of passion. The Kickstarter campaign encountered some tragedy along the way, but both Stewart Weick (RIP) and Lester Smith (the translater) as well as the original writers, Ricard Ibáñez and co. have done the English speaking audience a service for making this great game available for the first time.
[5 of 5 Stars!]