The Pharaoh Readme.txt includes the following:
"DATA LIMIT REACHED"
Pharaoh has limits on how many buildings, walkers and point-to-point walker routes it can handle. These limits are quite high, and you should never encounter them in the missions that we included with the game. Some of Pharaoh's "Custom Missions," though, let you build much larger cities, and you might reach one of these memory limits if you build a city of 35,000 or more people. If you do see this message, your only recourse is to resculpt your city for greater efficiency. Populations in excess of 50,000 are possible if you build very efficiently.
It's possible that the Pharaoh Readme.txt file is not included if both Pharaoh and Cleopatra are installed from a single CD.