Here's a short piece of advice that's good to keep in mind: monsters need room to move. Even novice adventure writers know that you can't fit 12 orcs into a 10 foot-by-10 foot room. But with a dizzying array of monsters, most of which are presented with only a single standalone image in a bestiary or monster manual, it's easy to overlook how BIG many monsters are. A purple worm may seem like a good underground threat, but it's so big it can't fit in many tight subterranean tunnels and really needs an giant vaulted cavern to fight.

I recently wrote an encounter location with 15-foot high ceilings, and only at the last minute realized that a few of the denizens are listed in the Bestiary as 20 feet tall. How uncomfortable for them! (In that case, I simply increased the ceiling height rather than pick smaller foes, because I'd already written so much about those taller creatures and how they interact with everyone else in the area.)

When mapping areas, as a rule of thumb, each monster should have no less than four times its space to move around in a fight. Add into that at least 4 squares to move around for each PC--a minimum of 16 squares for an ordinary party of four--and you need to either (a) keep your rooms a pretty good size, or (b) use smaller or fewer foes, to have your fights allow for fun movement and action.

Another personal story, about a Paizo product issue I don't think anyone has yet noticed. I was asked to write a section for Heaven Unleashed that would involve a new type of archon about to come out called an exscinder archon (basically, a heavenly censor). I didn't have the rules for an exscinder archon, but that didn't stop me from writing up a lively treetop stronghold, complete with a map. But just after I'd submitted the entry, I finally got to see the stats for the exscinder archon--and it's Huge in size! There wasn't any room for it to fight in its own treetop fortress! I frantically wrote to my developer, saying that "one square = 5 feet" on my map should be changed to "one square = 10 feet." My developer instead tried to shrink that particular archon in size, creating a disconnect where it's listed as Huge in one place and Large in another. But the fault is mine; asking about size should have been one of the first things I did!

Now, sometimes the point of an encounter is to constrain the combatants, whether as a hindrance (for example, to force the PCs into tight confines) or as a tactical benefit (for example, to allow the PCs to defend a narrow doorway against a horde). But these should be intentional, not accidental. Or else your archon can't fit into its own house!