I completed the third requirement relatively recently, by breaching a cavern through the ceiling.
And my entry:
Build this:
http://www.strategycore.co.uk/assets/images/games/hommv/HOMM5_PC_haven_town_wide_016_1600.jpgIn short:
A huge stone tower, several dozen tiles wide and long.
A circular basilica on top, right on the edge, with an optional statue. If you want to dedicate it to Armok, leave a hole in the center of it and drop goblins there.
A circular building on the opposite side of the tower (also meant to be a basilica), with tall stained glass (gem) windows.
Several (five on the image) towers near the main one, of varying height, connected by bridges on increasing levels.
Several smaller towers attached to the main tower's walls, separated into zones (crafting, living quarters, barracks, stockpiles etc).
For easier construction, you may leave towers hollow except for the surface/living area.
Additionals:
'Balconies' jutting out of the main tower, with extra living areas.
A stone wall around the tower, 2-z, with drawbridges as gates.
Guard towers on said wall.
A moat around the tower, or a full-fledged river (or just build it next to one, as it is on the picture).
Megadwarfbonus: a town inside the walls, of two-to-three z-level houses of wood and stone.