After I managed to create the Construction Spline, finishing the level was only the matter of time. 
First, using the reference plane and character guide, I decided about the size of the Maze corridors. Second, in the most time-consuming process of all the ground level was created. After I had the lower level walls, I was simply duplicating the parts I wanted to be higher and moving them to the top of the other walls.
Creating the Maze presented in separate stages
Finished level prototype