Prior to the release of Skyrim, Todd Howard stated that there was a lot of underground areas in Skyrim's cities, citing the Ratways as an example, however once the game was released the ratways was the only such area.

I think it would have been great if cities all had secret undergroundf areas and passages that could be utilised primarily by stealthy characters. Solitude for example has massive potential for this due to it's location on a cliff, so much could have been built into the areas of the cliff directly below the city. I just think it was an opportunity missed. Thoughts anyone?

I agree there should be access to the sewers in Solitude.
I think this may have been planned, as you hear the rushing water as you pass over the gratings, but was never implemented.

Raven Rock has an interesting underground.