while playing Final Fantasy XII in order to enter this certain city I had to summon belias and it opened the door. There were no clues at all for that.
There was a poem kinda thing talking about a giant to open the door as I recall actually.
I think the best FAQs/guides are the non-spoiler ones that just give out the solutions to moments like the one you pointed where people get stuck. It's really not fun when you missed something (or the game is just being unclear) and you end up stuck.