Where did the narrator meet the "knight-at-arms"?

The speaker of the poem meets the knight-at-arms standing by a lake. It's a pretty depressing scene. The sedge—a grass-like plant that grows near water—has withered, and no birds sing. The poor old knight looks rather dejected as he stands there, "alone and palely loitering." And with good reason, too. He's had the profound misfortune to be seduced and abandoned by a beautiful enchantress who routinely takes unsuspecting knights to her magical grotto, only to discard them like so many used tissues. The knight-at-arms is feeling so utterly miserable that he needs to spend time in an environment that reflects his dejected mood. That's why he stands there by the lake, where the sedge has withered, and where "no birds sing."
https://www.poetryfoundation.org/poems/44475/la-belle-dame-sans-merci-a-ballad