The editor now has the functionality to define a zone nicely, create possible encounters, define what creatures can spawn in them - along with all the relevant creature information. The app now picks all that up in a way that makes sense, and it all seems to function correctly for the most part. There's a few glitches, but mainly things seem to work as expected where I've properly refactored the code at least to move off placeholders.
Basically, I get to start working on some logic again in the app for creatures to start behaving the way I'd think they should - wandering about, attacking the player and so on - and using the underlying creatures to understand what it can do. It's a great place to close up Sunday night with.
I do still need to update the world-builder interface to be able to paint the zones in somehow in a meaningful way, possibly also tweak the spawn-tiles in some way, so that creatures only spawn on defined tiles, but that's in the future right now and I'm okay with it.