I am notorious for beginning to draw some adventure environment on graph paper, only to abandon it half-way through, on account of the map being visually uninteresting, or my losing interest in the adventure concept. That is why I like published adventures: I don't need to worry about drawing the dungeon, and there are lots of adventures that are adaptable to whatever type of campaign I am running.
My preference is to run adventures where the maps are both interesting and realistic. By interesting, I mean the following: there are lots of choices for the players to make; many of the environmental details are revealed on the map rather than in the related text; and the map is both functional and artistic. By realistic, I mean the map describes an environment that could exist from a structural perspective. I'm no engineer, but I hear rumor that certain construction principles must be adhered to, if you want a building or an underground tunnel to be structurally sound.
One of the problems I have faced with published adventures is that the buildings or dungeons don't make a lot of sense, from an engineering perspective. One of the worse offenders is Castle Ravenloft. While I love that adventure, much of the design of the castle didn't make sense to me. I kept asking myself if someone would have actually built that castle.
My point here is not to be critical of Castle Ravenloft. Rather, what i'm trying to say, in a round-about way, is that when it comes to drawing maps of castles and dungeons, I would rather crib from existing environments, particularly if those environments are also visually interesting.
Here is an example -- Castle Tamworth, in England. That is an actual castle, which was built and modified over several hundred years, and is both interesting and structurally sound. It has lots of staircases, (five, on my edited map, if you count the one I added that goes down into the dungeon -- that staircase is behind the secret door, and below the stairs that go up from the main banquet hall) that lead up and down to different areas of the castle. The castle also has several relatively self-contained areas, allowing different monsters to co-exist on the same level, with a minimum of interaction or conflict.
I suppose my title was a bit disingenuous. This post is not about drawing your own monster-infested keep, but about using examples of existing castles or dungeons as the basis for your own adventure. Clearly, those of you producing your own commercially-available materials cannot use existing maps, unless you have permission or the maps are in the public domain. But surely those of us that are creating home-brew adventures, for non-profit gaming, can do so?
So if you are planning to create a home-brew adventure, why not do a little digging in the library or on the internet, and find some interesting dungeon or castle maps, that you can use whole-cloth, or that you use as inspiration for the design of your next adventure.