Thinking a bit more on my convention game exploring the Outdoor Survival map using the rules from Original D&D Vol-3 (the DM's booklet) -- the following are the rules for stocking Castles (p. 15-16). Note that in these rules the only classes are Fighters, Magic-Users, and Clerics; so for my game I had to think about how to replace the Clerics with Thieves (consideration follows page scans; recall that for me, basins are caves, and buildings are castles):
A few comments: Note that the "occupants" are all either "name level" or "one below name level" in their respective classes (as usual, with Clerics as an outlier: they're either "lawful name level" or "chaotic name level"). Note also the weird wording on fighter/wizard alignment: they're either "hostile to adventurers (die 1-3) or neutral (die 4.6)" -- I read that first option to mean "chaotic", although obviously if your PCs were Chaotic you might as well make the hostiles Lawful for some action-packed gaming.
Anyway, as mentioned above, I wanted to strip out the Clerics and replace them with Thieves (more in-line with the Fighters and Wizards in terms of levels and alignment distribution, in fact). So in the first table, my last two rows look like this:
The monsters I used were almost all from Sup-I Greyhawk, but I think that's acceptable since it's where the Thieves themselves come from (the elves in the top-right are Hero/Theurgists, i.e., 4th-level in each class). For the second table I gave Thieves a 25% chance of having a Magic-User, level 5-8 (same as for Fighting-Men).
Open questions -- Per the text, "Occupants of these castles will venture out if a party of adventurers passes nearby", so as to make certain demands for tribute/treasure/toll, as indicated for each. One question is: What men do they bring with them? -- Initially I presumed just the castle leader himself with the Guards from Table 1, but in hindsight it seems a bit suspect that the castle ruler would jeopardize himself in such way (e.g., instead of using the castle itself for protection; perhaps that's good for the game, or perhaps they should bring a host standard men with them?). Second question is: What should Thieves do for analogous challenge/demands? -- I tried to come up with something, and frankly couldn't see anything in-flavor except for Thief-types to lay in hiding within their castle, hoping to ambush unwary intruders (which seemed to make sense until my PCs went to explore said castle and I had to make up reasons for how hundreds of men could remain hidden at once). Maybe you've got better ideas for those reactions?