Medieval Castles As Adventure Locations

I've been searching around, trying to find a suitable nexus floor plan for a mega dungeon entrance, without much success. However, in the course of my wanderings, I came across a website with a plethora of imaginary castles, many designed in the Harn-esque fashion of Castles of Harn. Sadly, the author of the website passed away in 2006. Someone has maintained the site, presumably in tribute to his creativity and imagination. I did a google search for "castle floor plan", and came across several other castles, including some really huge modern homes.

I like to use real castle and catacomb maps upon which to base my own dungeons and adventure environs. Doing so gives me some assurance that the environments are structurally sound and believable. Now, the castles from the above site are not real, but they are designed in such a way as they could be.

What I also like is when the layout appears to have been added to over time, giving the map a labyrinthine feel. This tends to give me an opportunity to have several areas that are more difficult to access. If the map has secret passages, staircases and the like, so much the better.