As for the core topics. The History Extension Course is quite different from the Prelim History Coruses. It consists of two components - Part I: What is History? Part II: History Project
As for Part I - it depends what your school wants to use as a case study. For Part II - Students choose their own topic in which they will conduct an investigation on. (This project may be completed partially in your own time)
History Extension is based more on the methods in which historians conclude their theories. It involves analysing written sources and evaluate whether the historian is impartial in his/her writing etc. Your chosen topic for Part II should be about a historical event that has caused debate among historians.
Well, I hope that gave you the basic information about the subject. Sorry for any typos and grammatical errors ^^" Hope to see you in the History Extension Forum soon.
[Had to divide this message into two because it was two long ^^"]