Map Design

Not only aesthetical, but also technical issues must be considered when designing a level. A BSP tree level contains of surfaces that intersect each other. The edges and vertices are not taken over by WED, but calculated from the intersections. The advantage of this concept is that you can overlap blocks - the invisible intersecting parts are clipped off by the build process, and new edges and vertices are automatically placed at the intersections. This makes rendering much faster, because the invisible overlapping parts of blocks aren't rendered (other engines would just render all blocks over each other). The disadvantage is that small angle differences can lead to large vertex position differences. If surfaces touch or intersect at a very small angle, inaccuracies of only 1/100 degree can move the joining edge position by several pixels, which is really visible in the level.

For this reason, don't use extremely narrow, elongated, or acute blocks. Do not create blocks with edges smaller than 2 quants. Always try to design your map with as less surfaces as possible, and use as thick blocks as possible. Sometimes you can't avoid surfaces touching at small angles - but even then there's a bad way and a good way to do it. For instance, if you have a sloping road over a hill (side view):

Bad design: uses long thin plates, and small angle inaccuracies in the marked joints will be visible as 'gaps' or 'steps' from above.

Good design: 3 blocks with 13 surfaces instead of 5 blocks with 20, and angle inaccuracies, even if they occur, won't be visible.

The BSP tree creation will 'snap' all vertices to integer positions. This can produce problems with T-junctions - a place where two block edges form a 'T' - when the junction vertex is not at an integer position. In this case the vertex displacement to an integer position can leave a visible gap or seam between the junction and the non-displaced crossbar of the 'T'. This is a common problem in BSP tree levels. The map compiler tries to convert as much 3-way T junctions as possible to 4-way 'X' junctions that aren't a problem in that reagrd. But it can't catch all. There are two solutions: a) have no T-junctions, and/or b) have all vertices snapped to the integer grid in your level.

Some more level design hints:

Use the snap to grid function whenever possible. Blocks that are aligned to each other, and have perfectly vertical and horizontal edges in all three windows not only look better - they are also compiled and rendered noticeably faster.
Each level should also be surrounded by a sky box. A sky box is just a very large hollowed cube with a sky texture (or any other texture) assigned. The level sits inside this box, like the office.wmp level.
Use the Detail flag for small blocks that don't obscure visibility or need collision detection.
Keep your map small. The closer to the center of the map a block is, the more accurate it is rendered. When you really need a large map, like for a city, use blocks only for gross structures like buildings, and use entities or detail blocks for details. Otherwise, visible errors can occur at large distance from the origin. They can be avoided by setting the d3d_nobsp flag in the engine, but better avoid them by design.
For all non-visible surfaces - like the bottom side of ground plates or the sides of water blocks - mark the None flag. This will not only compile faster, it will also save valuable texture memory that would otherwise needed for the shadow maps.