When does the Trophy Room open?

Only once, right after the final wave is done. Everyone heads for the Pipe Control Room, where all switch lights must show green to enter the Trophy Room.

What does completing all five challenges give?

The MKB42H | Volter | Precious weapon skin, available only during the time-limited 2023 Halloween event.

What happens when the final wave countdown runs out?

Everyone spawns in the boss arena to fight the final boss, so the trophy room must be done before that.

Specimens & Bosses

KF2 Castle Volter: Unlock the Trophy Room (This Belongs in a Museum)

During the time-limited Halloween event Last Hans Standing (autumn 2023) on the map Castle Volter, one of the five challenges was to unlock the castle's trophy room. This guide, which originally came with a video, describes the timing and lists all five challenges.

The trophy room

The Trophy Room opens its gates once only, right after the final wave is done and everyone heads for the Pipe Control Room. All switch lights there must show green to enter the Trophy Room. When the final wave countdown runs out, everyone spawns in the boss arena to fight the final boss, so plan to finish before that.

The five Last Hans Standing challenges

Completing all five gives the weapon skin MKB42H | Volter | Precious, only obtainable during this event.

I could do this all day: kill Hans Volter on 5 different maps. See how to force the Hans spawn.
Weekly Ending: complete the Weekly on Castle Volter.
Luckily they're not clones: find ten Castle Volter collectibles (Hans Volter plushies). See Castle Volter collectibles.
This belongs in a museum: unlock the Castle Volter trophy room.
A Fine Day at the Mountains: complete wave 15 on Endless Hard or higher difficulty on Castle Volter.

FAQ

When does the Trophy Room open?: Only once, right after the final wave is done. Everyone heads for the Pipe Control Room, where all switch lights must show green to enter the Trophy Room.
What does completing all five challenges give?: The MKB42H | Volter | Precious weapon skin, available only during the time-limited 2023 Halloween event.
What happens when the final wave countdown runs out?: Everyone spawns in the boss arena to fight the final boss, so the trophy room must be done before that.