Episode 38 of The Trigan Empire
The Secret of Castle Doum
aka The Secret of the Locked Room
Het geheim van de Abandon Burcht (Dutch title)
The Secret of Castle Abandon (Dutch title, translated)
Written by Mike Butterworth
Illustrated by Don Lawrence
It is reprinted in Volume 10 of The Trigan Empire—The Collection (the Don Lawrence Collection).
It is reprinted in Volume 4 of The Rise and Fall of the Trigan Empire (Rebellion).
Plot summary (Dutch)
MediaFire via British Comics Compilations