Tagmeth had been claimed once, then abandoned as blood feuds and foreign wars thinned the ranks of the Highborn and the great houses began to gather their strength in keeps farther south. Now frost-blackened brier roses scrambled in and out of Tagmeth's shattered walls and owls roosted in what was left of its rafters.
– Torisen's narration, Dark of the Moon, "Chapter 2: The Hell Hunt"
Tagmeth is a ruined northern Riverland keep, on the eastern bank of the Silver.
It is possible that Tagmeth keep will be restored in the future, given that in "Stranger Blood"—a short story that is set later than any of the novels—contains a Highborn named Kracarn of Tagmeth. It looks like The Gates of Tagmeth with entail that.
Tagmeth was originally an ancient Hathiri fortress, which like all the keeps, was itself built on the site of an even older Merikit hill fort. After the Riverland was seceded to the Kencyrath, Tagmeth was restored by Kencyr, but then abandoned again.