If the surface-worlders wish to take our cities, we will first choke the tunnels with their bodies and make them crawl through the groans of their dead.
This map shows us an ancient watchtower taken over by drow and adapted to suit their own needs. Indeed, they have completely left the surface structure to its own collapse.
An important feature to note about this dungeon is that there are no light sources other than what the party bring with them. If the drow realize that some members of the party are utilizing darkvision, they will make judicious use of their racial spells to level the playing field. Creatures in darkness, magical or otherwise, must make an Intelligence check with a DC of 10 (or equal to the spellcasting save DC of the drow who cast) it in order to not make a misstep and plunge into the waters below.
Traps feature prominently here. The majority of them, the ones leading to the rooms with arrow slits, simply deposit the victim straight into the rushing river below. Particularly nasty GMs can have swarms of quippers or piranhas waiting for unfortunate adventurers.
The traps across the main bridge work in devilish tandem. The bridge features two carefully-counterweighted sections that meet in the middle forming an almost perfect seam. The seam itself is impossible to detect unless a creature is within 5 feet of it. The front section, the section closest to the entrance, is only 5 feet long, while the next section is 10 feet long. Either section will swing down if they support 200 lbs or more.
The secondary trap on the bridge is a set of swinging spiked logs carefully hidden amongst the stalagtites in the ceiling. These logs are triggered by a pressure plate just behind the first stairs up to the bridge. A delay mechanism releases the logs 1 round after the pressure plate is activated. Not only do the spiked caps of these logs do damage to anyone slow enough to be caught in their arc, the weight of these logs is sufficient to knock adventurers into the river.
While all this is going on, the adventurers will be forced to march single-file across the bridge while under constant fire from the low walls at the far end. Drow heavy crossbowmen work in teams, with one drow loading while the other shoots, allowing a withering hail of fire. This also lets them fire, albeit more slowly, at independent targets when time comes to retreat.
The teams securing this outpost also use specially-modified bolts that feature a broader, flatter head, causing more damage AND staggering targets when struck. This can cause assailants to stagger back into the swinging logs, knock them off the bridge, or at the very least slowing them long enough to get a few more shots.
If this wasn't frustrating enough, the low walls give the crossbowmen cover, and form a barrier that is too high to easily leap or climb over and too winding to even sprint through without taking a few more shots.
Rounding out this gruesome gauntlet are the arrow slits carved into the rock walls. These windows have been carved in alignment with the natural fissures of the caves, making them nearly undetectable even as crossbow bolts start raining out of them. At the very least these slits give the crossbowmen three-quarters cover, and can even (in the hands of a cruel GM like myself) make them simply untargetable.
Assuming adventurers have weathered that bridge of death, they are now faced with a short series of twisting corridors filled with traps. Just past the first doors is a short hallway that features not one trap but two, fitting nicely into the theme of redundancy that defines this dungeon.
The first trap, activated by any weight being placed on the pressure-sensitive floor, fills the room with an odorless, colorless, toxic gas. This gas is heavier than air, giving it the benefits of a) affecting creatures smaller than Medium size first, and b) keeping it contained to the lower initial levels of the dungeon. When the trap is activated, the hallway fills with the gas in 3 rounds, and it spreads out to the maze behind the door in the next 5 rounds. A creature must make a new saving throw against the gas every round they start their turn in the affected area or move into the affected area. The gas disspiates after a minute.
The second trap is on the door at the end of the trapped tunnel. After a lever is thrown, any attempt to open or interact with the door triggers a fusillade of poison-tipped darts to erupt from the walls and ceiling of the whole hallway. After the first attempt, any failed attempt to unlock, disarm, or open the door triggers the darts again.
Further along come a set of alternating half-stairs spiraling up. For GMs wanting to add some verisimilitude and historical accuracy to their games, give disadvantage to melee weapon attacks against targets higher on the stairs than the attacking creature. This makes a perfect spot for a hold-the-line last stand for a drow trying to give his compatriots time to set up their defenses.
The remaining traps in this dungeon simply drop the victim into the water, as discussed before. The only exception is the trap between the commander's room and the priest's room; this is instead a magical trap that bestows some horrible magical effect on its victim. If you don't mind a bit of overkill, the power word spells are excellent for this.
Now on to the final ledge and bridge. Adventurers looking to vent frustration on the crossbowmen that caused them such grief from the safety of their arrow slits will be doubly disappointed; first because they will need to find the secret doors (that lead to trapped hallways) to even reach the arrow-slit rooms, and second because these crossbowmen will have already retreated to cover further back.
Cover in this case means a return to form: cleverly-concealed arrow slits providing enfilading fire, and low walls behind which the two-men teams can put their training to use. No traps on the bridge this time, but the stairs in the center count as difficult terrain and thus make it difficult to charge the crossbow positions.
Once adventurers finally make it to the large natural span, the reserves hiding behind the doors to either side charge forward to relieve their crossbow brethren. Here, the focus is not particularly on outright killing, although their targets are likely to be quite softened. Instead, these forces will attempt to toss or push assailants off the ledge, sending them to some watery doom off the map. My current favorite watery doom is a tremendous underground waterfall, but that's mostly because I'm a sucker for fistfuls of falling damage and creatures with Swallow.
If adventurers make it through that hell, they've passed the hardest point. Forces on the bridge will attempt to alert the rest of the cave once the battle is clearly lost.
It becomes the GM's choice whether the priests in the chapel flee or take position next to their altar, and whether or not G'umb'o the peg-legged cook fights or flights. More seasoned adventurers can find the priests guarded by a drider, animated statue of Lolth, or some other horrible monstrosity, and any drow footsoldiers seeking shelter in the chapel will have the sense to take cover behind the pews and attempt to surprise invaders. Should he try to defend his domain, the cook can use hot pokers, pans of boiling water, and the exhausting heat of his oven to give his killers something to remember him by.
RUNNING THE ADVENTURE
Like other encounters, this adventure buckles under the weight of higher-level adventurers. That's definitely something MicroDwarf is going to push against in the weeks and months to come. Perception check and Constitution saving throw DCs can only get so high before they become largely impossible to pass except for the few characters that have Expertise or use feats (ugh).
Characters who can fly or swim easily will be able to bypass a significant portion of the dangers of this dungeon. If you want to try to counteract that, try filling the open airspace of these tunnels with the thick webs of that common drow ally, the giant spider. Not only will this present a sticky (hah!) conundrum for characters who want to fly, it might also give disadvantage to ranged attacks from characters that aren't used to shooting through it (because I'm all about stacking the deck). Having said that, intelligent defenders will be quick to flee and get reinforcements if they intruders blowing through their defenses.
Interestingly enough, the distances between the important ledges is juuuuuuust past the maximum distance traveled by the most common short-range teleportation spells. Funny, that...
For those of you running Dungeons & Dragons 5e, this dungeon is quite well suited for tying in to the Rage of Demons storyline. Poor intel leading the party into an ambush they are completely incapable of dealing with is an excellent reason to throw the party into this horrible meatgrinder and into the prison cells of the drow. In addition, the duergar variants presented in the appendix make for excellent alternatives for the suggested drow as defenders.