I think I've got it (more or less)...
"The dread that haunted the great Tower, and fear of the glooms of Fangorn that lay behind it, might protect it for a while, but if it were unmanned and neglected, as it was in the latter days of the stewards, that protection would not long avail."
It's about Orthanc, somewhere in UT, but I haven't found it yet. Is this just narrative, or is it really said by a person?
Edit: I've found it now, It's in U.T, " The Battles of the Fords of Isen", in appendix (ii), and as I suspected, it's just narrative.
Obscure indeed!