Build 2 Small Pyramids to automatically research Town Phase.
Build 2 Large Pyramids to automatically research City Phase.
Pyramid costs adjusted to attempt to approximate the resources and time necessary to "research" the phase techs as other civs do, though the exact costs are subject to change for balance.
The Kushite phase techs no longer increase the Civic Center territory effect--the Pyramids do that now, with their "Necropolis" aura (exact territory effect subject to balancing). Build them in your own or neutral territory. They have no territory decay, but can be captured.