Going from my Castlemourn Player's Guide, Castlemourn is:
1) A complete setting, in a vein similar to FR and GH. It is a bit smaller, at least what is initially being produced, at least the map in the PG is ~16" by 10" at 100 miles per inch, or 1,600 miles by 1,000 miles, say from Waterdeep in the northwest to Amn in the south, and out to Thar in the northeast and Turmish in the southeast. And then, about half or mroe of the map in Castlemourn is water, with nine major islands... so even less area than that.
2) It is Greenwoodian High Fantasy. Names like Glamryn, Orthmael, Baerent, Khaladorn, Arshroon, and so forth, exhibit his style. The name "Castlemourn" comes from the fact that history was lost "when the castles fell," ~360 years ago, and no one has any memory or knowledge of what came before then, even the elves who still live from that time. There are many small kingdoms and city states, lots of "lost" and "debatable" lands, and plenty of ruins from before the castles fell. The cultures seem pretty much RPG Euro-Medieval derived standards.
3) The races are altered versions of standard races. Dwarves, for example, all believe life is an illusion, so have trouble seeing through illusion magic. Elves all glow in the dark, so there are very few elven thieves. Gnomes tend to be specialty mages of all kinds. Golaunts (goblin/orcs) are tough, spirited, and barbaric. Halflings make great thieves and, for whatever reason, can make magic items at 75% of the normal experience cost. Thaele seem to be a mysterious wandering people, apparently related to vampires and/or lycanthropes (there are few details on them in the PG). The one PRC included in the PG is the Waymaster, essentially a master merchant/swordsman. So trade, it seems, will be as important in this setting as in FR...