Or a heavy defense.
If I were to set up a steampunk city defense,
I'd have a thick wall around the city. On the wall would be track. On the track would be a war train, two engines on either side, with heavy, heavy armaments. The train would be a mobile turret. City guards would be able to reposition themselves with the help from the train.
If the city was to rebel, the train would sit at top of a gate, and crush any attempts to escape the massacre coerce the populous to an agreement.