I also wanted a bit of realism, not so much keeping to the lego economies of scale but more of a realistic scale. OK, the walls arent the 4-6 studs or so deep they should be but I've tried to design some realism into the structure.
Firstly I didn't want it to be a big boxy affair I wanted it to evolve, I didn't want a flat baseplate, I wanted the structure to fit in with the landscape. Which I did.
I started by building the landscape first. It wasn't built completely randomly and some parts of it were built with what was to come in mind, but in general I let the hill/cliffside build itself. The castle I wanted to build in stages. Most castles (at least the really cool ones) are built over a number of years - tens to hundreds. concepts, needs, architecture changes over time, so I'll build my castle in discreet stages. At the end of each stage I should be left with a complete castle, just bigger than the last. I had Chepstow Castle in mind when building. perched on a cliff edge, the castle was stengthened and enlarged over hundreds of years, a small keep to start with, then courtyards, barbicans and fortifications added over the years. So we start off with the Main Keep.