How to make a Simple 3d lift platform for Legacy
By Big Al of Doom Heaven
 

Look at the example wad called platform.wad.
The two switches will raise and lower the floating platform.
These could also be walk over triggers if required.
 
1 First create your 3d platform you would normally for Legacy, (sector 4 above).
I assume you already know how to do this.
 
2 Split your control sector into 3 sections (sectors 1, 2 and 3).
 
3 Make the floor/ceiling heights in sector 2 to be where you want the platform to be at its highest point. In this case floor = 128 and ceiling = 136.
 
4 Make the floor/ceiling heights in sector 3 to be where you want the platform to be at its lowest point. In this case floor = 32 and ceiling = 40
 
5 Make the floor/ceiling heights in the sector 1 (sector that controls the floating platform) to be either the same as in sector 2 or the same as in sector 3. Depending if you want the lift to start at the top or bottom. In this case I am starting it at the top so floor = 128 and ceiling = 136.
 
6 Tag sector 1 with the next free tag number, in this case 2.
 
7 Also put this tag number on your trigger linedefs. In this case lindefs 5 and 6.
 
8 Change the linedef's special flags to be of the "elevator types". In this case I have made linedef 5 type 234 (switch repeat elevator Down next highest level) and linedef 6 type 230 (switch repeat elevator up to next highest level).
 

That is all there is too it.
Pressing switch on linedef 5, lowers sector 1 (platform control sector) to the heights of sector 3.
Pressing switch on linedef 6, raises sector 1 (platform control sector) to the heights of sector 2.