You know that anything written by Ed Greenwood is going be a lavishly detailed immersion into high fantasy, and Castlemourn showcases his abilities (through the excellent MWP) admirably.
Castlemourn marries fantasy with one of my other favourite genres - the post-apocalyptic setting. Three hundred years ago, there was a cataclysm which destroyed the previous shining age, causing the gods to cordon off the realm. As such, none are permitted to leave, although some brave (or foolish) souls still try; and are never seen again. Don't let the idea of a contained setting fool you though; there is more than enough to keep players interested and enough political intrigue, open warfare, exploration and adventure to engage even the most experienced group.
There are all the mainstays of D&D in terms of character races, including two new ones - the Godaunt and the Thaele who do add a certain 'flavour' to the game by their presence and all the classes are present and accounted for. The only new class is the Buccaneer, and this is interesting enough (and balanced enough) to make it an attractive option (and who doesn't want to play a pirate).
There is also a host of new mechanically-flavoured items (including the ubiquitous Prestige Classes), but it is all very well written and presented.
With Ed Greenwood at the helm, you know the book will include the practicalities of daily life, and he doesn't disappoint. Calendars and crops, foods and festivals are all covered in varying degrees of detail and give GM and player alike a real insight into daily life - something I've always admired from his Forgotten Realms work.
Given that this has been reduced to $4.99, I cannot think of a single reason why this shouldn't be on every fantasy gamers' shopping list. Even if you are a 4e player, the narrative content alone will be useful. If you are looking for a new setting for your next campaign, give this a chance. You'll not regret doing so.