For the swordsmith guild, my guess would be that the hidden resource makes it available as soon as these places reach the "large castle" level. The guild can also becomes available in all castles as far as I know, when you build enough swordsmen there (I had it after about 10-12 units built).

Other possibility is as mentioned above, it makes the guild more likely. I've never played whatever factions own damascus or toledo in the early game, so does anyone have any feedback?

Third possibility is it does nothing at all.