Much to my surprise, I found his lecture on the civilization of Mesopotamia extremely interesting.  

A Contrary to expectations, his lecture on ancient Mesopotamia was the most fascinating of all.

B It was at his lecture on the civilization of Metosotamia that I realized how fascinating the subject is.

C I was fascinated by what he had to say in his lecture on the civilization of Mesopotamia though I hadn’t expected to be.

D I hadn’t expected him to lecture on the civilization of Mesopotamia, but he spoke remarkably well.