Description Economy: Keys

A key contains the descriptions for the locations on your map. 

You need to be able to see what is going on in an area at a glance, so you do not need to stop play to figure it out. Else you create empty waiting time, and invariably your players start to check their cell phones or discuss other things and you lose their immersion. 

What do you need for your homebrew adventures?

 You really only need the items that you cannot easily improvise.

Wha tthat is of course depends how well you improvise. But in my experience, that means, you only need some sketchy notes, one liners, to jog your memory of what you had in mind. I try to make the key fit on the map. That failing, I try to make what does not fit the map fit on one page that can be open next to the map. Having the monsters right on the map makes it easy to see who is in hearing distance of whom, and might come to aid if a ruckus is heard, or how long it will take for help to arrive. No need for any tactics or development descriptions.

• Monsters, Traps and Treasure. Who or what is there, what do they have?

That is really it. Gary's Greyhawk Castle had no more, he made up all the rest. He also did not put the key on the map, to be able to modify it. But that is only needed if you are running an open table megadungeon, where multiple groups adventure in the same locale and if the map is very dense. Normally you explore an adventure only once, and having it locked in on the map in open space is fine.

You can do a bit more to help you in your minimalist notes, things hard to make up on the spot:

• Descriptions of riddles, unusual and complicated traps or new magic items.
• Stats for key NPCs. These can be re-skins, or stat blocks you develop. 
• New and unusual monsters (again, rip and reskin as much as you can). 
• DCs and damage/effect on secret doors, traps or challenges, unless you use standard values based on your assessment how hard it is or set those on the fly based on party strength.

Then, there is flavor, optional stuff that you probably have an idea for in your head or more easily can make up or decide on the fly, and that would be more important to write down if you publish for others:

• Room Names. Well-stocked larder. Pompous Bedroom. Decrepit Privy. Ad-lib the rest, refer to your DM screen for trapping ideas. If you use symbols for furniture you have that detail already. 
• Factions on the map - which groups are there, and how do they stand to each other, what are their goals. From these conflicts, adventure and solutions to problems spring. Can be a small sketch of faction names with relationship arrows.
• History. Don't write a novel of backstory that the players will never learn about. Leave out all that "used to be" and "GM background" stuff, it contributes nothing. But if you decide the dungeon was originally built by dwarves, use that to flesh out descriptions - there may be murals of dwarves, the secret door may only be 4 foot high, the password to activate it must be spoken in dwarven etc. If the place has been repurposed from a church to a bandit headquarters, there will still be the altar, holy water fonts etc, likely put to new uses.
• NPC roster. If it is a detective adventure, then a table of the personae damatis and what they know and are up to is good idea. If it is an organized outfit, a tally of forces available will help speed things up, as may a plan of battle in case of an attack, i.e. who does what then (though you can make that up on the spot, too). A simple schedule of oprations might help, if the PCs scout out the place for a heist or raid, just don't overdo it.

You can see an example from an old 2e adventure I wrote here. Everything is right on the map, the extra key lists the main antagonist, his henchman, a major new monster, specific areas, and a cross-section of the dungeon. I should have compressed all that to one page.

How should keys look like for published adventures?

A lot of the discussion how keys should be written centers around published adventures. This is treated well by the Alexandrian's the Art of the Key. Older adventures tend to have a "wall of text" to provide all the detail for each location. After all you bought the product for that detail. Furthermore, most adventures are bought for "lonely fun" only to be read, never played, which further lends support to a novelesque narration. But to make it playable, the infomration must be organized differently, so that the DM can quickly see what is going on: 

If you have a good map, it already should indicate light, locked doors, and dimensions of the room including height. If not, say so up front. You do not want give a full room description, only to find that the door to the room is locked or the characters cannot see. Then:

Start with an name that gives the overall function of the room. A name may be all you need in mundane rooms, especially if you have a DM screen with lists to help you make up details. If your room is generic, like a kitchen, hallway, dungeon, bedroom or guard chamber, a single added word can help to give it some character and inspire description. 

Follow with a paragraph that says what is immediately visible. Old adventures used boxed text to indicate this, but it does not need to be in a box, italics also works well. If there is a prominent monster, feature it prominently in the description, as this is what the characters will naturally focus on. Set items for which you have more detail to follow in bold.

Next, list effects that players can discover immediately or that happen as they enter (saving throws, perception checks, ambushes). 

Then provide follow-on paragraphs or bullets headed by the bolded terms from the description, with detail for when the characters investigate or interact further.

Sometimes there are separate sections calling out monsters and developments -- things like nearby monsters that will hear noise. Be wary to not use a room template, where you fill in the obvious just to fill the template. You often can decide who will hear battle, or make a check for it, without needing to spell it out.

Here is an example of a room description following these guidelines (assuming entrances, light, shape, height can be taken from the map):

Antechamber. Double doors flanked by life-sized bas-reliefs of manticores. Against the south wall stand 3 skeletons with curved swords and shields. Carvings adorn the walls.

• Double doors DC 15 to pick lock
• Manticores roar loudly with magic mouths if someone listens or casts a spell at the door, alerting troll (room 8) and mage (room 10-12).
• Skeletons. AC 15. Attack if disturbed, or if anyone tries to open the doors without key.
• Carvings show soldiers besieging and burning a city, flaying and enslaving people  

For published adventures that do not use this format, you will need to put in work to make them usable at the table. At least highlight what is immediately visible when entering a room (underline, or color).

Do you need the monster stats and hit points in the key? 

This may have been possible in 1e, where that was just one line of stats, but in 5e it would be a large block eating up a lot of space. Instead, where possible just refer to the Monster Manual, maybe noting a different skin, or exceptions (max hp, wounded and half hp, blind, acid damage etc.). Use standard hp, or eyeball variations, if you do not want to waste time and space for pre-rolled hit points on grunts. 

With multiple different monsters this approach becomes more difficult as you have to peruse different pages, and flip back and forth. Pysical bookmarks help, or look up the monster on your phone or tablet online to have a separate "sheet". When we play online, googling it with "monstername 5e" ususally is faster than anything else. 

If you are trying to publish the adventure and make it easy to run, this will blow up the key to several pages. In that case, it might be worth to include a minimized combat vignette for the monster right there on the page for ease of use.

Adversary Rosters

The Alexandrian advocates creating a table of all the monsters and NPCs on your map, with a second column where they can be encountered. This is useful to more quickly respond to player activity, if you are dealing with at pages upon pages of descriptions in a fully written out product. 

For hombrew, if you only have a 1-page minimal key consisting of one-liners of monster and treasure to begin with, or if you have them right on the map, this is not necessary. You already can do that.