An unknown castle or small fortress somewhere although, because it is quite small, it is possible that this is a house which has been designed and constructed in the style of a castle. It is also possible, of course, that the angle the picture was taken gives the impression that it is quite small and the rest of the structure is hidden from view. There is a flag flying on the top of the building, but I couldn’t make anything out from zooming in when the image was scanned.
Update: Bruce Cunningham has pointed out that this is Pendennis Castle in Cornwall (see comments below) so I have altered the title of this post accordingly but left the text above untouched. Thanks very much Bruce!
The date of the picture was not marked in the slide mount, and the nature of the picture makes it impossible to guess the date.