If you want to read up the design principles on which the PTL is based see here.
This documentation aims to be of help of you want to write your own client for polymake or your extension using polymake C++ template classes.
In the Related Pages you can find information on the design principles of polymake, efficiency considerations and other helpful stuff.
Moreover, you there are documentations of the Namespaces used by the PTL, Classes implemented by the PTL and the most important Files in the PTL.