Right, found it in "Unfinished
Tales", "Part Two: The
Second Age", IV "The History of
Galadriel and
Celeborn"; page 238 in my 1986 paperback reprint of the original edition, but on page 2
58 of my 2020 40th anniversary special edition hardcover. That's more like it.
"Now
Sauron's immediate purpose was to take
Lindon, where he believed he had the most chance of seizing one, or more, of the Three Rings; and he called in therefore his scattered forces and marched west towards the land of
Gil-galad, ravaging as he went. But his force was weakened by the necessity of leaving a strong detachment to
contain Elrond and prevent him coming down upon his rear."
a bit later, near the bottom of the page
"The army that was besieging
Imladris was caught between Elrond and Gil-galad, and utterly destroyed."
Now in my opinion, that seems to be a "defensive siege", aimed at preventing Elrond from breaking out, not an offensive siege with the intent of taking Imladris. For which purpose the "detachment" was probably inadequate, given its later utter destruction. But it is still not clear, to me at least, which specific form Imladris's resistance to a siege took. No description known to me gives the impression of it being a
fortress, as I mentioned above.
But then, all of this Second Age action took part over 4,700 years before the
War of the Ring at the end of
the Third Age. My guess is that quite a few
Elves had left
Middle-earth in the intervening millennia, so any forces Elrond might still have had at the end of the Third Age must have been greatly diminished - and that is what would have to be compared to
Thranduil's
realm of the same time in the Third Age.