The Physical Book
The adventure is a 16-page, full-color, saddle-stapled book. Like all of Paizo's Pathfinder products, the production values of this item is really nice, something which is especially worth noting considering the book was given out for free. The art, especially the cover and the Wayne Reynolds' pieces, is very good. The cartography decent but pretty basic and, to be frank, rather boring - the rooms are almost all octagonal in shape, being differentiated only by the number of doors and the contents of the room.
The Adventure
The adventure, which is designed for 1st level characters, is nothing special nor particularly interesting. The basic premise is that a recent earthquake has provided access to an ancient siege tower which gives the PCs the opportunity to explore. There's a little more going on in the tower, but that at its core is what the adventure is about.
As for what's inside (remember I promised this would be spoiler-free), there's a decent enough reason provided for what's taken up residence within the tower, but for the most part the adventure is pretty much a room-by-room dungeon tower crawl. The PCs move from room to room, killing (or maybe talking) to what's inside before moving on to the next room. While the arrangement of the tower means it's not a linear crawl, it's still a crawl.
The Other Contents
Of the 16 pages, the adventure only takes up pages 2-9. Pages 10-15 contain pre-generated PCs, each of which also serves as a preview of the upcoming Paizo's Advanced Player’s Guide. These are pretty cool and not unexpected since part of the strategy behind Free RPG Day products for many companies is to provide something that also doubles as advertising or preview material.Each of these characters includes a small, iconic-style portrait, and a complete write-up which is quite nice although of limited utility to anyone not actually wanting to run the module using the PFRPG.
The Verdict
The Master of the Fallen Fortress is a decent enough adventure for a free product but nothing I would recommend paying money for or going out of your way to buy unless you're a die hard completest. That said, Paizo deserves some kudos for producing such a good looking free product.