Marshtide Watch is an Alliance outpost north of the Temple of Atal'Hakkar. In the questline that you get from Joanna Blueheart, you lead a campaign against the Horde in neighboring Stonard, in a 3-part phased Questline mirroring the horde's (besides the ending of course). Eventually you conquer Stonard but choose to let it stand and not "Stomp it to dust." In the Horde's questline you repel the alliance.
Interestingly, the max-level guards who spawn at Marshtide Watch are Hunters, not Warriors, using abilities like Fire Trap and Serpent Sting.
This was a site for one of the great final battles in the Second War where the Alliance of Lorrderon defeated the Horde. It is also a place where the orcish Horde gathered together before they were known to exist in Azeroth.