It has some great things and some horrible things.
Best thing is that it does a decent job of suggesting how to build pretty much anything you can imagine. Tower surrounded by a hurricane? Staffing, defenses and accoutrements for said tower? All there. But how useful is this all? Depends on you. If you are a DM in search of ways to flesh out an evil wizards tower or a friendly temple then its spiffy. If you are a party of adventurers bulding your stronghold, its very useful as a one-shot and continues to be somewhat useful if you want to upgrade. All good possibilities.
Worst thing is the editing. Omigod, I dont think I've seen a professional product released in this state. It's full of stuff like, "Undead cannot be created or summoned into this space. Undead created or summoned in this space gain +2 hit points per HD." (p75) Huh? Some stuff doesn't even make that amount of non-sense, its the sort of sentence fragments you get when you start cut-n-pasting too fast in the ol' word processor. Abysmal.
Slightly bad: The "Traps" section is reprinted from Song and Silence. If you already have this book (and I bought it just for this very section) then you now have two copies. Sad face.
More than slightly good: The sample strongholds at the end. Six good, solid examples to get your mind churning about the practice of making fabulous buildings/dungeons/towers.
Unknown, untested: There are a lot of magic rooms, magic items and magic effects for your stronghold. Some of them worried me about how balanced they might be; I dont think I'd rubber-stamp any design PCs brought to me without carefully checking which items & zones they had chosen. Until I playtest though, I cant know for sure.
Overall, the good outweighs the bad. I dont think its a must-have book, a playgroup could easily get by without it; but if you are interested in some solid guidelines for buildings, here you are.