I loved Batman: Gotham by Gaslight. I found the concept of Batman in the 18th Century to be intriguing and the story line was excellent. The characters are all names that a Batman fan would be familiar with, but it is the way the characters are worked into the story that makes this a great animated film.
The settings felt a bit like London in the 18th Century, and it could have well been London, I noticed that they seemed to be a reference to the Charles Dickens character Bill Sykes, along with a bit of mixture of Sherlock Holmes as well.
All in all, a great movie and worth purchasing if you are a animated Batman fan like me.