[3.5] Advice on dungeon construction for a new DM?

[ComposerSuzuran]

I'm currently doing the prep work for a campaign for a group of friends, and all of us including me, with one exception, have never played a tabletop game before. (The one exception is a long time 3.5 player, thus the choice of system). I feel like I have a fairly good handle on mechanics, however, I lack experience. At least I know the players fairly well.

For the first leg of the adventure, I was planning on sending my players through a low-threat dungeon to let everyone get a handle on the system and kick off the plot (which I'm keeping loose and free-form on purpose). However, I'm finding the construction tips in the DM's Guide too vague to be able to do much with them. Does anyone have any general advice on throwing together a good low-level dungeon, and/or mapping utilities I could use?

Some background info if anybody needs it:
The group is four, potentially five players. We're planning on running it through Roll20 and Skype chat.
Everyone's starting at level 3, plus whatever they get from the intro solo adventures I'd like to run. The two I've talked to about their characters already want to be a Sorcerer and Cleric, if I recall correctly.
Plot-wise, the world's mostly custom-built with a very Renaissance Italy feel, and a focus on clockwork. The plot hook is the little magical engine that runs the clockwork infrastructure in the town they're starting out in has been stolen and must be recovered from a nearby set of ruins. The "boss" of the dungeon is a small group of Mysterious Cultists™, mostly low-level Adepts with a Cleric leader.
We're using only Core, PHB2, and the "Completes" for sourcebooks.
Will provide more information if needed.

[Emperor Tippy]

Tippy's process for creating relatively random dungeons.

Step 1: Decide how many equal CR worth of encounters you want to take place inside the dungeon. You generally want about four per day spent inside, so if this isn't a deep delving expedition figure 4. This could be six CR 2 encounters for an ECL 3 party or one CR 7 encounter (just for example)

Step 2: Decide how many discreet encounters you actually want to occur (i.e., how many times will Initiative be rolled).

Step 3: Look through the Monster Manual and find challenges that fit with what you decided in the first two steps. For example, maybe the first encounter is a Black Bear that is resting in the mouth of the cave (a CR 2 encounter). Remember that traps can be encounters as well and your Rogue might like one.

Step 4: Once you have figured out all the encounters that you want, start laying out the dungeon. Decide whether you want it to be pretty straightforward or whether you want the PC's to be able to really sneak around and have lots of options. Straightforward tends to be simpler to make but it pretty much limits the party to combat encounters while multiple entrances, hidden passages, environmental features, etc. tend to be a bit more work to design but allow the players more leeway in how they handle the challenge.

Step 5: Grab a piece of paper and a pencil and make a quick sketch of what you are thinking. Should this room be round or square? should the passage to the next room be straight or bendy? Etc.

Step 6: Grab Graph paper (or a computer program that lets you lay out grid maps) and now turn your rough sketch into a finished dungeon map. Mark all the encounters, traps, treasure, etc. on it. This is your master copy.

Step 7: Make a copy without any of that stuff marked on it for each player and one extra for the party as a whole.

Step 8: Use another piece of paper, post it notes, or the like to cover over the parts of the map that the PC's haven't explored yet.

Step 9: Play.

[Gnoman]

I prefer to start from the opposite direction, personally. For me, the first step is always "What was the "dungeon" originally for, and what is it?".

Did your cultists erect or excavate their own structure, or did they "acquire" an existing one?

If the former, what was their goal in setting it up? Did they focus on defenses, or is the entire structure filled with various ritual chambers, prisons for sacrifical victims, and such.

If the latter, who did build it, and why? Was it a watch-tower, or an iron mine, or a hidey-hole for orc raiders? How did the builders set things up, and how did the cultists adapt it for their purposes?

[Kudaku]

Try to have it make sense - start out with the original purpose of the complex.

If it's a tomb or a treasure vault, it would make sense that it has numerous locked doors, traps with no easy bypasses, summoned guardians, golems, undead or other creatures that don't age, need rest or sustenance. Conversely, if there are living creatures in the complex then they'd need a source of food, water, air, and so on.

If it's an inhabited dungeon then that'll reflect in how it's designed. If there are traps then there'll either be a way to bypass them or they'll be keyed to only target outsiders. There probably won't be many locked doors when you get inside the complex unless they protect especially sensitive areas. Try to add in the little touches that makes it feel "lived-in". A kitchen, the subterranean equivalent of an outhouse and so on.

[Seharvepernfan]

Decide what the ruins were, and what happened to them. This will determine the layout and contents. If they were partially destroyed in a magical battle or something, you might want to put in some collapses and weakened construction, for example. You want to make rooms/chambers/hallways/etc that make sense, given the location.

Personally, I try to avoid the rooms-and-hallways design style, as it's boring and not particularly realistic - I tend to make dungeons/adventure-locations into one large structure (where the players can knock down a wall to get into a room if they need to, or go through a window and climb on the outside to another window), instead of a series of rooms connected by linear hallways.

Most importantly, your dungeon should have a theme (and maybe some sub-themes), which can be as simple as "Cave with a waterway flowing through it" to "ancient vertically-built fire-giant stronghold full of traps and constructs, now haunted by the spirits of dead treasure hunters". If it's just a bunch of stone-rooms with no purpose, your players won't remember it and it'll probably mess with their virisimilitude.

Then, pick a few types of creatures/enemies to throw in the dungeon, based on what it is and who your enemies are. At level 3, you might pick animals (rats/wolves/bears), undead (skeletons/zombies/ghouls), monstrous vermin, goblin bandits, etc. Place them around the dungeon, paying attention to how they can get to and fro for food/water/whatever, and how they interact with each other.

Figure out likely ways for the players to get around, and what kind of obstacles/hazards will be in their way. Don't be afraid to make them balance/climb/jump/swim/crawl. Put in difficult terrain, cover/concealment, darkness, usable terrain/obstacles (like being able to knock over a bookshelf or roll a barrel down some stairs). Give them several different ways to get somewhere. Consider the visibility, background noise, floor/door/wall/ceiling type (and how this can affect tracking and str-checks-to-break), non-dangerous critters (use speak with animals to question rats, or follow bats to find an exit), and smells.

Can they learn anything about the place or it's inhabitants with knowledge/bardic-knowledge checks? Gather Information? Divination spells?

Think about loot. Is it hidden? Is it laying about? Are there things in the dungeon that are valuable, if the players decide to take them? Are there any traps? What kind of containers is it in? How would the players go about getting the stuff?

What about the journey to and fro? Perhaps bandits follow them to the place, then ambush them as they are leaving with all their new-found treasure.

[Tvtyrant]

My advice is to always aim for cool. In clockwork Italy this probably means gear-driven mechs and retro-industrial environments. Having a dungeon where instead of traps you have to disable machinery like metal stamps and conveyor belts seems like a good inclusion.

[ComposerSuzuran]

Thanks, everyone, for all the advice. It's a load of help, and I think I'll be able to sketch this thing out now.
I was thinking these ruins were an old, decaying fortress. The above-ground parts are mostly collapsed, but the underground parts have been cleaned up, reinforced, and converted into a temple for one of my homebrew deities.