Hmm, that doesn't seem right. Thanks for the report! On a very quick trial run it's working for me (with both OUT and DOWN), but I'll take a closer look when I can. Were there any unusual circumstances? What browser are you using?
Aha, this might be a deliberate part of a puzzle. (Sorry, it's been a while since I wrote the game, so I couldn't remember it myself!)
*** SPOILERS FOLLOW *** - to decode rot13, see https://rot13.com
If you lead Achilles into the fort under circumstances — [rot13: jura uvf obbgf ner bss naq uvf Npuvyyrf' urryf ner ihyarenoyr] — you won't be able to leave until Hector kills him. When you try, you ought to see a message like ""Achilles dodges a swing and jumps in front of you, blocking your way out." Is it possible you were missing that message?
If you want Achilles to win, you'll need to solve the puzzle a different way.
What I remember is that I gbbx Npuvyyrf gb gur sbeg jvgubhg uvf obbgf, Hector killed him in a single strike, and after that, I decided to, for some reason, haqb hagvy Npuvyyrf fgvyy unq uvf obbgf ba. I went there with him again, and that's when I couldn't leave until Hector killed him, which seemed to happen at random. Sometimes Achilles managed to dodge his attacks, other times he got hit and died.
All in all, I think it might have happened because of my gratuitous and thoughtless use of haqb, but nevertheless I think it's an interesting bug.
(I'm cyphering the spoileriffic stuff in rot13 just like Robin has done.)