I last played Ultima V on my Commodore 64, so yeah, it's been a while for me, too, but I don't remember any cages from Shadowlords. All I remember is that glass swords could be used to drive them away (temporarily defeat them), but usually I just ran away from them if I happened to encounter them. I think the 'cage' bit may either be an error, or perhaps something added to a later port of the game. I doubt its veracity.