Campaign Setting Design: Triumphs & Mistakes?

[Zaydos]

I'd really like to hear more about how your parallel primes worked. How did they communicate? Move around? Handle resources rights?

There were no spells to directly communicate, but there were some naturally occurring portals (short term ones were common, most lasted months or years) with long term ones (stable for generations) being hubs of trade, and ancient artificially made permanent ones being major trade centers. I didn't actually get into resource rights in detail, but had some notes that there were conflicts over them between the powerful but isolationist empire of one world and some of the countries of the pseudo-European world, and the third world was divided between a lawless monster land (a red dragon was taking control of the region) and the overstretched empire of a gold dragon which was actively trying to avoid expansion because its ability to control the empire is stretched too thin as is.

There were more details (the worlds were sealed off from each other ~12000 years ago and while the seal was weakening for thousands of years it was within the memory of some entities for example the dragon emperor had finished his expansionist phase before the connection became easy).