The microtubule cytoskeleton is an active network that adopts distinct architectures at different cell cycle stages. Using microscopy-based bottom-up reconstitutions and computer simulations, we build minimal self-organizing microtubule networks that mimic cellular architectures, focusing especially on the role of microtubule nucleation and moleculer motor action for network organization. Our goal is to uncover physical design principles of the architecture of mitotic spindles during cell division. We present here such physical principles that ensure that at metaphase minus motors focus microtubule minus ends into exactly two spindle poles whereas plus motors organize a central array of antiparallel microtubules.