In this tutorial I will, assuming you have prior knowledge of DEdit and know how to build a basic room with lighting and a seal, explain how to create a climbable ladder.
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First you must select a texture for your ladder's visual part, try 'WorldTextures >> Details >> Ladder3'.
Now create a brush, make it thin and as high as the ground to the point where you want it to end, centre your marker on the brush and bind it to a translucent world model (right-click > bind to object), go into the properties of the object and look for the 'chromakey' option and set it to true.
Now make a brush using the chrome or invisible texture, located in the worldtextures folder, make it from the ground to a few units higher than the ladder itself (so the player can get up and off the ladder safely), make it the same length as the ladder and give it a fair depth (enough to engolf a player, whatever looks right to you).
After doing this bind it to a volumebrush >> ladder object (right-click > bind to object), in the properties of this object you may modify how quickly a ladder can be climbed and other things.
Now process and run your map, climb your ladder and become king of the world!
-Rasirez