Terrain wall, exterior walls and half walls

[Aruba]

Hello there I have a small problem, tried to make a terrain wall all around my terrain, no problem with that but when I try to add a fence gate or door it won't add it to the terrain wall. I tried to add a half wall instead of the terrain wall and on 3D the half wall looks like it is 4 feet above ground?! Any ideas are welcome, thanks.