A state-of-the-art 1 THz cascade backward wave amplifier was fabricated in the frame of the FP7 European Community OPTHER (Optically driven Terahertz amplifier) project. The huge theoretical and technological effort to realize an amplifier at a frequency of 1 THz has created new knowledge and advancements in the field. A double corrugated waveguide was adopted as the slow wave structure to support a cylindrical micro electron beam. The technique developed while working with the challenging dimensions, the assembly of the parts, and the results of the project represent a big step toward the reliable realization of THz vacuum electron devices.