Don’t model with brushes
Some things are better created with brushes, as part of the world. Others are best modelled-up and added as a prop or entity. Typically, the more detail something requires, the greater the likelihood it should be a model. If trying to create something intricate is too difficult with brushes, then it’s a prime candidate for a model.
Understand that the engine lets you create geometry down to a 1-unit granularity (at the vertex). Models don’t have this limitation and can be much more complicated.
Speedy from the start
Everyone wants a map that runs smoothly and maintains a good level of FPS, so work to maintain this from the very start. If you’ve got an idea for an area or location, think about how expensive it will be, and consider how to reduce any repurcussions it will have on the rest of your level. This soon becomes second-nature with enough experience making maps and having to deal with the problems they throw at you.
HINT brushes are terrific tools, but use them sparingly and only when there is no alternative. The visibility calculations are generally superb, so if something’s not as fast or optimal as it should be, chances are it’s because of your design choices.
However, don’t limit yourself by this. An expensive area will often distract from a cheap area nearby, so use this to your advantage. You can afford to push things a little… just not too much.
5 common technical mistakes
Forgetting clip brushes, allowing players to jump out and exploit the map (see also: forgetting to turn the right visgroup on) Not orienting player start points sensibly Forgetting that all-important additional model and/or texture. Leaving in entities intended for testing purposes. Not testing.
Work to the grid
There’s a grid system for a reason. Use it wherever you can, and stick to 16⁄32/64’s for small-scale/interior work, and 128⁄256/512’s for large-scale/exterior work. This helps keep things neat and tidy, and the tools act slightly nicer. Objects and props rarely need to be grid-aligned. It isn’t 1998 anymore.
Work against the grid
The grid system is great, but can inadvertendly force compliance and repetition. The real world is not conformist. Sometimes, but not always - by a long shot.
While it’s tempting to make everything a power-of-2 or multiple-of-64, break it up. Put some non-standard lengths in there (multiples of 4 or 8.) Not all vents are created to the same dimensions. Not all ladders are aligned to 32-unit boundaries. Not all floors are perfectly flat. Even just a subtle change to the width of, say, a corridor, can help break the monotony of everything stemming from the number 8.
Work with the grid
Considering the above, if you have bits that are perfectly grid-aligned and bits that aren’t, you’ll have inadvertendly highlighted that the world is artificial. The grid is there to help you create a world. It’s your job to hide its existence from the player.