In Bulmer's "Keys to the Dimensions" series, there is a dimension-spanning empire headed by the Contessa Perdita di Montevarchi, centered on the dimension/world of Irunium. By ruthlessly controlling "Porteurs" (those who can find and use dimensional portals) the Contessa has access to the resouces of many worlds. As the series builds, more people (many from Earth, some natural Porteurs) are drawn into contact with this empire - which is inherently unstable, and building toward a climactic battle as dissidents and disaffected make their escape and combine forces.