Settlement Plans and Buildings?

Questions;

Faction or Culture Specific? and if the plan is culture spec can the buildings be faction?

Is ti possible to have toher arrangements of walls and walkways than the typical ring wall? Like say have a castle in the middle with towers which you can walk up on top of? Or even planked walkways above and through the city?

Would it be possible to have essentially a huge dome over the city? like so the whole thing is inside a hill and the only way in is the tunnel gate?