Originated in PMU 6, Archford is a region accessible in PMU 7, though one must use HM Surf from the Exbel Penninsula and beat Crescent Islet near the South Outpost Island first. Archford has its own dungeons, non-player Pokémon, Housing Center, and town. You can fly down to Archford from the lower left area of the sky. It is an overgrown plant region with many dungeons and new Pokemon; as it's the last region that can be unlocked, it is considered the hardest of the four available regions.
Spark Cave (PMU 6)
The original way to get to Archford in PMU 6 is by taking the Ferry from the Winden Docks.