At Some Forgotten Door by Doris Miles Disney (MacFadden-Bartell, 1967)

Bought at the closing down sale of one of my favourite bookshops:

 

STARK TERROR NAILED HER TO THE SPOT

They stood facing each other, her enormous eyes reflecting the fear for her life--the greatest fear she had ever known.

Neither of them moved or spoke, testing the unique bond between them, the bond of murderer and intended victim.

He broke the deadly silence. "If you'd listened to reason..."

She saw his hands clench and unclench.  She could almost feel them at her throat.  She saw him stiffen with the resolve to get it over with.

Hetty hurled the lamp into his face.

... And I don't blame the heroine for being scared of the house - look, there's a giant head growing out of it!

Seriously.  This was sold as crime fiction, but it's much closer to being a gothic romance.  There's a sinister house - check - an orphaned heroine - check - and vague intimations of something being wrong before anything actually happens.  There's a charming young man whom the heroine insists on marrying even though everyone warns her against him.  And it's not until the very last chapter of the book that the heroine goes exploring and discovers a secret room filled with dead bodies.

Lastly, At Some Forgotten Door is set in the 1880s.  This came as quite a shock to me when I opened the book, because there is nothing on the cover to indicate that it is a historical novel!