Seed Sky Castle (Seed天空城, Seed tenkū-jō) is a location in Rune Factory 5.
This is area is also called SEED Floating Fortress in the North American Release. This area has some challenging monsters which are around Level 88 so prepare yourself appropriately. Be sure to wear decent equipment, items and be sure you have leveled Ares, or Alice sufficiently to around Level 90. It is possible to clear this area with a lower level and a good party of townsfolk / monsters.
This area is unlocked after defeating the boss on Cloudheim 3F. After completing this area, you need to do an event with Lucas which will unlock a portal to Calamity's Edge. That portal is found near the SEED Ranger Outpost in Rigbarth.
They are numerous monsters in this area, this area is great for earning experience.