I'll go through it step by step: first i drew a wall, pulled down one side of it to make it sloping, then i drew a line, using the 3d polygon, tool from one top corner of the wall to the other, next i drew line (3d polygon) from the middle of the bottom of the wall vertical to just under the top of the wall, then i used the connect combine tool.
To get the snap for the middle of the wall bottom you need to 'activate' the corner snap points first, a middle point is then automatically created....
All this done in left isometric view and with the smart points constraint active (the smart edges constraint wasn't the one, sorry)