So I was wondering how folks here approach this? Do you start with monsters and add traps and puzzles? What about adventuring day concerns?
When I created the Catacombs of the Lizard Kings (for my home Tomb of Annihilation campaign), I started with the theme &
making the map. The theme was a dungeon used by lizardfolk priests of Semuanya to house the never-decaying corpses of Lizard Kings so that the "possessing demonic essence" could not leave them once the corpses decayed – thus, preventing the demonic spirits from going back to the Abyss, only to return in another Lizard King. So it was both a tomb and a containment site. I also knew I wanted it organized around a water feature, so some amphibious/aquatic monsters would be good, probably some undead. When figuring out the inhabitants, I created a big messy list of monsters that might fit, including:
Giant slug*, dracolisk*, skeletons or mummies (of lizardfolk), allip, shadow demon, ankhegs (jungle variety), pterafolk, quasit, guard drake, basilisk, giant subterranean lizard, giant crocodile, black abishai, hezrou, hydra, etc.
Then I compared some of my ideas to other parts of the adventure, and quickly realized that pterafolk & basilisks & giant crocodiles could feature elsewhere, and that a hydra was already present in another dungeon in TOA. I also added black pudding (as a place to dispose of innards from mummified corpses & a trap) and babau (vaguely reptilian looking) to the list as I worked. Be flexible. New monster ideas/connections will emerge as you work.
What I ended up with was this: