I managed to upload my game before the deadline and everything seemed to work fine locally. When I then tested it through the browser, enemies were not spawning as expected. This appears to be because of the loading time of the levels, adding a brief pause between levels, and using _on_enter_tree instead of _ready seems to fix this.
I hope this is an acceptable reason for a late upload, the original zip file submitted this morning is still listed on the site, but not playable. If not, thank you anyway.
(No other creatures were harmed, or poisoned to create this update!)