Are you planning to upload some content to Quadropolis - Great! Read on!
The Cube Wiki article Distributing Maps contains the original text of this guide;
This deals with the specifics of Quadropolis postings.
It tries to prepare you for providing well rounded off content to the community and avoid any unwanted tags, maybe even get you a starred content one. ;-)

I just want to share some knowledge and give some tips on mapping. This is for novice mappers as well as for people that are already familiar with Cube editing. For learning the basics of how to edit maps, refer to the readme.html.
Some opinions on certain topics might be influenced by my personal taste and views. I don´t claim this were the ways to do it, but it´s the way I do it and what I find important.

Since I have never made a singleplayer map (I like multiplayer better) I can only talk about making multiplayer maps here.
I will divide this into sections, which are

1. Layout/Performance
2. Gameplay
3. Design (detail, mapmodels, light and stuff)

1. Layout/Performance

Starting a new map, it is always good to have some basic concept or idea in mind.

I found this most helpful, and haven't seen him explain it this well before and didn't want it to get lost in the comments of node/3184 'Why's there fewer starred submissions these days?', so I politely quote it here.

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eihrul | 2011-10-15 01:40
The problem is that the mapping community is shooting itself in the foot. Quadropolis built mappers from the ground up, but now tries to operate at a level of sophistication that does not allow mappers to get feedback and grow. As has been said, new mappers are shoved away when they want community and feedback. Its standards are too high and simultaneously the wrong standards. I have a rough set of standards which I apply to maps for inclusion, which are really a balance of many aspects. If you only focus on one of the aspects, to the neglect of another one, the map is unlikely to get included. If you do a passable attempt at all of them, the map is likely to get included, even if it is not amazing. The bar is not amazing, the bar is balance. They are:

1) Performance. If the map is 400K verts because you spammed lots of little details everywhere, it is probably not getting included. 200K is more or less a sane top-end for big Hallo-sized maps. Spamming lots of water planes is almost always a no-no, especially in combination with spamming lots of geometry, since they multipled eachother. Try to stick to at most 1 water-plane in view at any one time.

2) Texturing. Texturing matters not just for appearance, but so the player can actually see where the hell they are going. I'm not trying to single anyone out, but if I am handed a map with very complex flow, and all hallways look the same because it's one solid gray mass or a such a noisy mass of random textures that I can't remember a pattern to them as I am running by, then the flow of the map has become more theoretical than actual. And, well, neither looks nice either. I may as well be running through a dark and twisty maze only to be eaten by a grue. :)

3) Lighting. The comments for texturing above apply to lighting, though with some slight difference. Too much noisy lighting with lots of different technicolor lights or almost-moire-pattern shadows from competing lights can make it hard to see and it just looks ugly. Even a simple sunlight with some skylight with some occasional accent lighting can be good enough for a lot of maps. Most people just try too hard here.

3) Geometry. I am actually rather forgiving of blocky detail, and people tend to overfocus on geometric detail to a fault. Spamming lots of noisy detail really tends to make maps look worse in general, and actually detracts from nice texturing and lighting, especially the texturing. You need to work with the cubes, not against them. Large details tend to work better than small ones, especially because at the pace people run around in Sauer your small detailing will mostly be ignored as people run by like cheetahs. Let the textures do the small detail.

4) Particles. Don't spam them. Sometimes particle spam can work in the right context, but usually it just looks a bit lame. It's not a deal breaker, but never the less, lame.

5) Mapmodels. Same caveats as spamming lots of small geometry detail.

6) Flow. Cramped tunnel systems or cramped hallways are bad. Too much verticality is bad. Too many teleports is bad. Too many jumppads are bad. Too open is bad (especially in CTF and capture), less so for FFA. Too many twists and turns is bad. The most popular FFA layout in Sauer seems to remain a hole with a ledge around it (complex). I think people overthink this one a lot as well.

7) Items. I think common sense mostly reins here, and that it is rare for people to really screw this up badly. Make sure items are available at appropriate frequencies according to their benefits. It is perfectly okay to omit the quad and health boosts and yellow armour in a map if you don't like their effect on gameplay for your map too, but generally all other items should be present. Quads/health boost/yellow armour very rare. Green armour kinda rare. Rockets/chaingun should be used judiciously. Shotgun/rifle/grenades a bit more available than rockets/chaingun. Health to taste. Pistol cartridges should only be placed only to insult/spite the player, but are not terribly useful.

8) Clipping. Usually I end up going in and fixing this myself on almost all included maps, but people never clip their maps adequately, so that the poor baby kittens escape and fall off the map and die. Save the kittens.

9) File size. The Sauerbraten download size is already big. Many people still have shit connections, and even if they don't, a huge download makes it less likely for impulse downloaders to try out the game because it will still take a long time to download. I don't care if you think otherwise. The download is not increasing 20MB for an FFA map, and even 5MB will make me skeptical. For 5MB, your map must be pure unadulterated awesome-sauce, like, better than almost every other map in the game. Smaller file size maps that are less a hindrance on the total package size are more likely to be included.

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Circle Template for Circles with even(blue) and odd(red) diameters:

One Way Cubes

So - we saw handy do these in his mini_arena
and then explaining their creation in a tutorial included in factory..
also SheeEttin gave a very quick how-to in his comment to the mini_arena-node.

I've decided to put this up as a real guide too though - credits do have to go to those two for the research though! Well done boys - that's the true spirit of this community!!

Entrants should follow the following guidelines for entries into the competition:

Today, I have release Version 1.0 of my Sauerbraten editing cheat sheet. You can download it from my site, or here on Quadropolis.

The cheat sheet's aim is to give both new and experienced developers a desktop help that they can have on hand when editing levels. Version 1.0 only covers basic editing, with future versions to include heightmap, entity and other config settings that users find handy.

Anyone is welcome to download, and contribute or make suggestions to the project, which forms part of a series of tutorials on Sauerbraten I am planning.

I: Thou shall not infringe on copyrights.

II: Thou shall not make gimmick maps.

III: Thou shall not post a map that took less than 12 hours to complete on Quadropolis.

IV: Thou shall not use the clip/noclip materials in thine first three maps.

V: Thou shalt not capitalize file names.

VI: Honor thy Aardappel and Eihrul

VII: Thou shall not create spawn points facing a wall. This includes both monsters and playerstarts.

VIII: Thou shall not use the smallest grid size to create an entire map.

IX: Thou shalt not touch the lighterror variable. Ever. I will break the face of anyone who does.

X: Thou shalt not be a douche bag on public servers.

Although probably not as fun as level design, making skyboxes can add a lot to the experience of creating a map, as well as the atmosphere and gameplay of a level. Skyboxes can be loaded with the "loadsky" command, followed by the path to the files. The "loadsky socksky/desert" command for example will load the desert skybox located in the folder packages\socksky. You can browse to this folder and view the six images that compose the panorama you see in game. This guide will focus on the process of creating a skybox with several software rendering packages.

Many of the Cube and Sauer skyboxes were created with Terragen. You can find a detailed step-by-step tutorial by googling for "Terragen skybox" and "Lloyd M". In Terragen, as well as in the other packages we're about to use, there are a few basic rendering parameters that need to be set for each of the six images so they can blend seamlessly. In Terragen v0.9 (the version used here), all of the parameters can be accessed from the Rendering settings window. Images need to be square, so first click on Image size and set both height and width to 128, 256, 512, 1024, etc. Click Camera settings and set Zoom / Magnification to 1. Below is the camera orientation, which you can set individually for each image before rendering:

HOW-TO play sauerbraten

This document is now unmaintained since wiki:Multiplayer Guide has more current information and more contributors to edit it :)
[2006-08-22] (updated link:2006-12-10)