The anvil cloudlands are only tersely described, and should, like many extraplanar or flying realm jaunts, provide just enough material and adventure for one or two game sessions. Some plot Hooks and Hazards are provided to make for a fun few hours of tabletop goodness.
The Anvil Cloudlands are designed as a flying realm with the 13th Age game system in mind, but could be adapted to other worlds. Depending on your game system and setting, the Anvil Cloudlands can supplement or replace one of the Inner Planes. In standard D&D cosmology, the Anvil Cloudlands can be a location in the Plane of Elemental Air that is a little more ripe for adventuring. In Eberron, it could replace Syrania, the Azure Sky, which isn’t as perfect as it at first seems. Making the Cloudlands a particularly large Demi-plane is also an option; Maybe Tanithar created the whole thing, pulling organisms from all over the multiverse for some kind of grand experiment, or his great magic just birthed them so he could have a place to put his Tower.