I somehow skipped book four so missed a few plot points. This one seems slightly sketchy on the plot - but as I missed book four I have to give that a pass at the moment. The Roman history it build on is good - I like how he illuminates Caligula and builds on what might have happened had he continued to reign. I also liked how he brought in why he was termed Caligula and all of that. Nice touch with the "just because they are from the past doesn't mean they are dumb" aspect either. Shame you didn't do a scene where the VP got some of his own medicine. :D
I liked Cato - although I'm puzzled as to why you didn't mention who he should have been. Macro was fun. Enjoyed the Senators too. Uncertain as to why Caligula didn't keep the official historian that came along around along with the guy he did.
Oh and I also learned that this author's brother is also a writer and he has used some of his brother's characters (Cato and Macro*) with permission. *I mentioned Cato before, and there is a historically significant Cato in Roman history - but I haven't checked yet to see if this is the same Cato.
There will be a re-rating once I've read book four. Consider this a pre-rating!