For starters, you get this product for OSE, 5e, Troika!, and Sharp Swords & Sinister Spells all in the same purchase! So kudos to the authors for that. The main book covers the keep and a host of important NPCs. The characters are all assumed to be young adults living within the Keep. This covers 72 pages. There are also some new monsters in abbreviated stats that can easily be used by any game.
The GM's Scenario packs vary from ruleset to ruleset. 5e is 43 pages and OSE is 55 pages. As expected these GM packs give you scenario seeds, the relevant players/NPCs, and has you go from there. The flexibility of this a crazy high. I can see an enterprising GM make this their central focus for dozens of adventures if not an entire campaign. If you don't want to do that then make it a home base for the PCs and have the occasional "stay at home" adventure. Given how well it is multi-stated use it as a means of moving from one game system to the next. It is extremely well designed.
Get it for one system, but enjoy it with the other three as well. This has made me want to look more into the Troika! RPG.
FULL DISCLOSURE: The author of this, Richard Ruane, is a co-worker and friend of mine, though I did not see that at the time I purchased this.