It reviews a number of proposals, not inconsistent with the known laws of physics, which have been made for habitable artificial astronomical megastructures.
Niven can conceive of four ways of generating artificial gravity in a spaceship: (1) centrifugal force; (2) adding mass, e.g. neutronium or a black hole (this would incur serious penalties in fuel consumption); (3) gravity waves; and (4) continuous linear acceleration to the midway point of a journey, followed by continuous deceleration.
As an alternative to Blish's idea of launching existing cities into space, Niven proposes a giant annular spaceship, which rotates to generate artificial gravity.
In that conception, a solid ring with a radius of perhaps 1 AU surrounds a star, and is spun like a bicycle wheel to provide centripetal force instead of gravity.
An Alderson disk is a platter of diameter similar to that of the orbits of Mars or Jupiter, with a star occupying a hole in its center.
[2] In a topopolis, a star is surrounded by a toroidal tube, which rotates around its internal circular axis to generate artificial gravity by centrifugal force.