Be more precise by setting the location of the vertex with numbers
While creating any of the 3d elements, plane, portal, cube or extruded polygon, after the first ground click, you can hold SHIFT + A and a modal will pop up to define the value of the height.
You can also set that value anytime, by clicking on the vertex while pressing the mentions short code.
0.- if you click on a red vertex "ground vertex" you can only set X and Y because height is always suposed to be 0
1.- When you change the XY of any vertex from a line it will affect to the rest of the vertex that belong to the line