The Moongolow bonus level is a reward for unlocking hyper mode for 4 levels. Hyper mode is unlocked for a level once you beat the boss that spawns at 25 minutes. Meaning to do this, you need this achievement:
You have to play Moongolow more than once to survive for 15+ minutes.
The first time you play Moongolow, just as the clock is hitting 15 minutes, the level will go into a special sequence, and you'll be teleported into a new area without any of your upgrades. You must run through this passageway to get an item at the end (the Yellow Sign).
If you die during the running part, you do not have to replay Moongolow in its entirety and can select this sequence on its own, called Holy Forbidden, from level select. Holy Forbidden will disappear from level select once you complete it.
Moongolow will continue to teleport you to Holy Forbidden at 15 minutes until you complete this sequence and get the Yellow Sign, earning this achievement:
Once you get the Yellow Sign, you can replay Moongolow, where the level will no longer end at 15 minutes.