Couple of important defense questions:
-wiki says that aquatic creatures can swim through fortifications(but land creatures can't walk though them? makes no sense but hey it's DF). Question is; how much liquid must be present? Does it have to be 7/7 on either side or will a simple 1/7 puddle suffice?
7/7 water will let them swim through. Creatures can't swim in less than 4/7, but if the water is fluctuating it may push them through anyway. 1/7 will not let them pass through.
-If you have a raise bridge that raises into a wall: will it act as an atom smasher squishing what was on the bridge against the wall?
Raising drawbridges have one edge that turns into a wall when they raise. Anything which is on that edge will be crushed. Anything which is on the rest of the bridge will be flung in a random direction. If a creature above a certain size is standing anywhere on the bridge, the bridge will refuse to raise.
-I saw a neat diagram that showed "fortification-floodgate-fortification" to protect it from building destroyers. Does this mean that a fore mentioned aquatic creatures can't actually stand in the fortification; only move through it?
I would not trust that diagram. I have not tested this, but I would expect an aquatic building destroyer to be able to destroy that floodgate anyway.