This won the World Fantasy Award in 1980, and very well deserved it was.
You know how fantasy books often start with a map, and then a plan of a castle? If they do, I usually look at them first and try to sort things out. The castle plan, in Watchtower makes a lot of sense, and gives quite a lot of information and sets up certain expectations, such that it's quite surprising when the first line of the book shows the place taken in war and burning. This isn't the last expectation Lynn violates.
There's a standard sort of story you might expect, about a watch commander of a watchtower who takes oath under the conquerer to protect his lord's son, and on the surface this is that story, but underneath it's something quite different. The world is very solid, the characters and their dilemmas are absolutely believable from the first second they're introduced. There's barely any magic, and the conflict is small scale -- retaking a castle, not saving a world.
I could wish for more fantasy like this.