*Always changing floor height when I wanted to change floor textures.

*Using special border textures between different wall segments and doorways.

*Being strict about texture alignment.

*Conscious use of contrast everywhere in a level between light and dark areas, cramped and open areas.

*Making sure that if a player could see outside that they should be able to somehow get there.

*Being strict about designing several secret areas on every level.

*Making my levels flow so the player will revisit areas several times so they will better understand the 3D space of the level.

*Creating easily recognizable landmarks in several places for easier navigation.