To generate a 3D environment, Doom engines extrude a two-dimensional overhead map of lines and vertices. This imposes some limitations: for instance walls are exactly vertical and rooms cannot physically locate above or below other rooms. Modifications are able to circumvent some of these limitations.
The Doom map format defines a level as a kind of negative space. A basic room is a series of one-sided lines that enclose a sector. Objects can move around inside the sector, and the outside is treated as solid impassable space. Two-sided lines sub-divide a space into smaller sectors which have their own floor and ceiling heights.
Since all traverse-able space is enclosed, a Doom map normally doesn't provide a view on an endless void like a grass-land or a star-field. However its simple enough to construct a view into space outside the map. First of all, a special texture allows any sector to render as if open to sky at the ceiling or floor. A simple technique can also transform a ceiling wall to look as if it is open to sky horizontally.
Horizontal movement or sectors is restricted by the map format, but floor and ceiling heights are free to change. This results in map designs making use of elevating platforms and vertical doors.
Hexen modified the engine to allow for horizontal motion of sectors under certain circumstances. Doom engines with this kind of feature can have swinging and sliding doors, blocks, etc..