I'm thinking the random encounter tables will be really small, probably only d6 or d8 tables, because there will be lots of planed encounter areas with more interesting stuff, and I don't want a huge number of random creatures running around.
So the creatures in each area should kinda be simple stock creatures and make sense for that level of the dungeon Exterior sections will have both night and daytime random encounter tables.
Like I said earlier though I want monsters to start to roam and migrate as the players unlock different areas. To this end the 6th entry (or 8th if that's what I go with) on each table will be a redirect to a monster from a random adjacent (and unlocked) area of the castle.
So say I'm rolling a random encounter for the Great Hall and I get a 6- which indicates the encounter is a creature from a nearby area. I then randomly determine its from the Ballroom. I roll on the Ballroom encounter table and get a 3- A dancing ghostly couple appears in the Great Hall. If I had rolled a second 6 it would have meant a roll on the Treasury encounter table. Using this method something could theoretically crawl up from the Crypts and menace the PCs out on the Battlements.
I'm going away for a bit and might not post for a week or so.