1. Make a brush covered with a ladder texture.
2. Select the brush and turn it into a func_illusionary.
3. Go into its properties and set Render Mod to Solid and FX Amount 255.
4. Create another brush covered with the CLIP texture, and place it in the exact same spot as the illusionary.
5. Create another brush covered with the AAATRIGGER texture, and place it 1 unit away from the ladder.
6 Select it and turn it into a func_ladder.
Done.