Quick guide for the collectibles achievement and how to unlock trophy room in Killing Floor 2.
Killing Floor 2 Castle Volter Collectibles and Trophy Room
Switch Locations
Outside near a couple cages 
In the bath area
On the balcony
By the courtyard in one of the door openings
In the hall with green light
Near an outside door
Third floor library
In a hall near a library door
Next to the trophy room door
Collectible Locations
I was able to find 4 in the library
Hiding behind a desk
Sitting on a shelf
Sitting on a couch
Underneath a desk
I was able to find 2 in the kitchen area
Hiding behind boxes and trash
In a bucket on a shelf
I found 2 of them outside
Laying inside an open crate
Sitting on the house
The rest I found randomly throughout the map
Hiding next to a shelf/drawer
On the pool table
Underneath the armored vehicle
Hiding behind a couch near a trader
Hiding on some broken stairs
Laying on a bed near a trader
Theirs probably a few more I didn’t find, but hope this helps.
