The full extent of the info I can kind on the "famed" warrior Elfrin is that he founded a small kingdom based out of a keep on the High Road in 1290 DR and that he died of a fever in 1305 DR. Does anyone have any additional info on either Elfrin or his keep and kingdom? I am not aware of any mention specifically of ruins of his keep on the High Road or in the Mere of Dead Men. The only hint I have that there could be something more is that George Krashos's timeline of the North gives Cloak & Dagger p.108 as one of the references for Elfrin's 1290 DR founding of the kingdom, but I cannot find this reference in Cloak & Dagger.
It should be page 107 of "Cloak & Dagger" and merely references Marune - which probably doesn't fit necessarily with the timeline reference. There's no other information that I am aware of.
-- George Krashos
"Because only we, contrary to the barbarians, never count the enemy in battle." -- Aeschylus
I think you may want to look in Dragon Magazine #70. There was a fort, just south of the Mornhaven Tower, that might serve as the location, and a jumping off point to discover more. That tower was identified as have been built and abandoned some time before.