The Winding Stair  (English, Electronic book text, Hodge Jane Aiken)

Juana Brett's hapless father has dragged her to England from her beloved home in Portugal. In Portuguese, she is not hindered by the stutter that plagues her when she speaks in English, and she is not at the mercy of her teasing stepsisters and icy stepmother. When Gair Varlow fixed his political machinations on her, he never expected to fall in love with her too. Pulling strings to get her an invitation back to Portugal, he knows he is sending her into danger. For Juana, nothing could be better than going home. But as the threat of invasion by France hangs over her return, she has no idea how completely the political intrigue has taken hold of her Castle home, and soon finds herself amidst terrorists and spies. Juana can trust no one, which is made all the more difficult when she is caught between the persistent courtship of her handsome cousin, and the draw she feels towards Gair Varlow. But all this is dimmed by the mystery and terror of the winding stair...This swift historical romance was first published in 1968.