At present, there is only one way to create a slope: give a line a special
of 181. Set the first argument to 1 to align the floor in front of the line
to the floor in back of the line. Set it to 2 to align the floor in back to
the floor in front (same thing, just the other way around). The second
argument is the same, except it controls the sloping of the ceiling.

How aligning works: Suppose you set the first argument to 1 so that the
floor in front is aligned to the floor in back. This means that the
height of the floor at the line you gave the 181 special to will be the
same as the floor height you set the back sector to in the editor. The front
sector's floor will then slope up or down so that the point in that
sector furthest from the line will be at the floor height you set for the
front sector.

Later versions will allow more control over slopes, including changing them
during play.

Using doors or platforms that are sloped/near slopes should work now.