Buck Ryan was a young British private detective who fought crime on the pages of The Daily Mirror from March 22, 1937 to July 1962 and was revived on August 3, 2015. Crooks, kidnappers and German spies were his meat. One antagonist, the lady crime boss Twilight, eventually reformed and became an ally and (perhaps) romantic partner of Ryan. The strip was created by artist Jack Monk and writer Don Ffeeman
The stories were told in long arcs, about four a year. Many of these adventures (140 of them) have been reprinted on the Comic Book Plus website. I don't have the date for the original strips for "Shadow Castle," but it appears ti be a later adventure because Twilight is an ally and not an antagonist.