Obsidian is something I've never attempted to play with much, other than the occasional farm and such. However I'd like to change that. Simply, I want to make my entire fortress out of obsidian. I've thought of a few different ways to do this, but I haven't quite gotten the mechanism down for doing it on such a massive scale as this. I know that I'm definitely not the first to do this, so looking for some tips from the veterans.
Which would be better, digging out the area where my fort will be then filling it or building up? Each adds it's own complexities, such as if the area above ground before I put the obsidian fort there, will it still be above ground after the obsidian? If so underground farming could be an issue, if not, above ground farming might be an issue. Also, if I decide to build above ground I'll need some ideas on what to make the design (I should note I plan on putting a full sized fort into this plan, so 150 bedrooms, dining room, etc. so it will need to be large enough). I'm already leaning towards building up, as if I'm going to do the obsidian, I might as well make it look cool too.
The pump stacks I'm cool with, although they will take some time to build above ground. However, I'm at a loss as to how to properly mix the water and magma. When I have done it in the past for farming, I've used a retracting drawbridge to just dump water on magma, however I'm not sure how well that method will work here.