Or when to know when to quit rolling dice.
Having created the entry in Part 1, I decided there should be a transition between there and the first dungeon area. ToAD has a table for that, which has many possibilities branching from it.
Transitions Between Dungeon Areas (Table 3-23) gave me 'A bridge over a river or chasm' and on the second column of the table a Hazard Trick.
Hazard Tricks (Table 3-92) gave me 'Venting steam, bad air or dangerous gases. Reasonable hazards for a chasm, so no river we have a bridge over a pit. And that gives me the idea that one of the other levels connects back in at the bottom of the chasm.
Wait a minute, I need a description of the actual area where the chasm is bridged. Back to Corridor, Basic Description of (Table 3-24) which gives me a 10' wide, 10'high natural cavern...with two unusual features!
Corridor, Unusual Features of (Table 3-25) gives me 'Large grates in floor' - must be in the bridge deck. And a Trap. Basic Mechanical Traps (Table 3-126) gives me 'Magentism'. As long as I'm here, I'll roll up the gas from the Hazard Trick on Gases (Table 3-128) which provides the information, 'Removes Oxygen' and 'Lies near floor, lingers'.
At this point I realized the process had gone off the rails, there were too many features being crammed into a minor corridor - and I hadn't even rolled up a description of the Bridge yet. But it had given me a number of inspirations to put together.
I decided to strip it down to the basics, a cavern crossed by a loose grating. Which immediately put in mind of driving over the old Jamestown Bridge to Newport, Rhode Island the first few times I went there for schools in the Navy. Let me tell you an open metal grating 135 feet over Narragansett Bay, if not terrifying, at least gives the driver an immense sense of relief to have gotten over it.
Leaving the entry through the opening under the frieze of the bridge, the party steps out onto a narrow ledge 5'wide and 10' long. A grating bridges a chasm.
The chasm is about 15 feet wide and it's roof is about 10' over the bridge. Based on what your torch illuminates, it's more than 30 feet deep. You get the impression that the chasm narrows in both directions, but the light doesn't illuminate the ends. The grating itself isn't fastened to the ledge but rests about halfway across the ledge. It's made of joined wood, with 2 inch members and 10 inch holes. Any characters moving at greater than half speed need to make a Dexterity (Balance) check or fall. Potential consequences of the fall range from dropping anything in hand to plummeting into the dark below.
Better illumination will show the chasm is approximately 45 feet deep. If they drop the light off of the left hand side of the grating, there is a 50% chance they will spot a doorway at the bottom (Perception check). If they state that they are doing the detailed examination while on or across the grating, they will notice likely handholds in the chasm wall nearest the Entry that can be used to reach the bottom.