The fall of the towers

From back cover Sphere paperback 1977:

The Empire of Toromon was the last hope and refuge of mankind

Sealed off from the charred radioactive wastelands by the radiation barrier, the Empire survived to face new adversaries deadlier even than the Great Fire. The Lord of the Flames, a force of evil devoid of physical substance. The berserk computer which guided the Empire's military complex. And an alien intelligence which crossed the abyss of space in search of new worlds to conquer.