I'm not sure which bonus level you are referring to, but idle has a few unlockable levels, so I'll just list all of them:
1) Hardened levels: unlocks after beating the regular version of the level
2) Challenge levels: unlocks after you unlock the Auto Conquer advancement (which also unlocks the idle battles)
3) Super-ascend levels (the levels after Europe Huge): unlocks once you complete your first super ascension after spending 100.000 AP in phase 4 advancements
A long long time ago, when the idle-only app was just released, there was indeed a bonus level in the app (a special version of Final Earth iirc), but that went away about a month after the release.
EDIT : Perhaps
https://www.warzone.com/Forum/509200-warzone-idle-app is relevant here?
Edited 4/9/2024 06:24:39