Sentinel Hill is a small outpost in Westfall and the last stronghold for the Alliance in the region. Most human players end up in this area at some point after level 10 and through level 20 or so. The area is located southwest of Stormwind and only has a few vendors and other amenities.
In addition to a mailbox, a few vendors and some repair options, it also has a flight path, an inn, and forge and anvil next to the lumber mill down the hill from Sentinel Tower. This is where all missions for the Deadmines and the surrounding areas are picked up from.
It is quite common for Horde players to stage raids on Sentinel Hill with the intent to either kill Gryan Stoutmantle or to provoke a fight from Stormwind City in an attempt to draw Alliance players away so that a larger attack can be made on the main citadel. Of course it is also jsut as common for some less than scrupulous players to try and get lower level players to flag so they can "gank" them for purposes of griefing them.
In addition to the road that leads up from the Goldshire area just to the right of the road from Sentinel Tower there is also a Gryphon master and roost here to provide access to other areas in the Eastern kingdoms.
There is a large contingent of The People's Militia found here and many quests associated with them.