The Altar of Peace is a monument given as a reward for completing the Pacifist Task. Like all monuments, it can only be built once per game, and provides 3 population and 400 score to the city it's in.
Using Mind Benders or retaliating after being attacked will not reset the timer.
Interestingly, the Altar of Peace is one of the only 2 monuments out of the seven that do not appear to be built all according to a main theme (Gates of Power are all gates, etc), with designs varying from a fish tank (Aquarion) to a rectangular prism (Ai-Mo). However, the monuments of Imperius, Kickoo, Hoodrick, Luxidoor, Zebasi and Yădakk seem to have a similar theme, being a variation on an archway with three or four legs, and a domed or curved top.
|Altar of Peace, Emperor's Tomb, Eye of God, Gate of Power, Grand Bazaar, Park of Fortune, Tower of Wisdom|