Day 4. I'm IN! Thanks to Gorobay's corroboration of my guesswork about the star pattern. The sequence is in fact N E L E K.
The stone slid aside to reveal a passageway ending in some stairs going up:
No doors that I could see, and no more appearing letters. I followed the stairs, and after some twists and turns, came out here:
It's strange. It's like a facsimile of something out of our history--seventeenth, eighteenth century? But not quite. That house on the left:
No doors. Windows, and peering in I could see dim shapes that might have been furniture, but no doors. Whoever built this had seen a picture maybe, or heard a description, but had no idea about the reality. Of course, if you can link you don't necessarily need doors. It was the same all the way round:
Lots of intriguing places to explore, if only I could get at them, but no way in from where I was. Till I found this:
and made my way up through a trapdoor into a wooden walkway that curved round the side of the castle:
which contained, halfway along, this. I'd found a way in.
I'll be working out the floorplan for the next couple of days, trying to sort out some basics like who owns this place, and making a list of things to be restored. Then we can really get going.