Ok, here's what I think I'm going to build for my entranceway (roughly). Tell me why this won't work:
-------------GG-------------------
___F_..................................F__
-----\p______HH__________p/
--------------GG------------
____________F__|||
Where GG's are grates, and H's are hatches. p's are pressure plates set to activate if water depth is greater than 4/7, and attached to the hatches. F's are floodgated on a manual lever. -- is normal stone tile; _ is dug out floor; . . . is empty space I put in to make the diagram fit on the forum.
Idea being, water flows in from the ceiling grate and falls down, creating a waterfall across the entranceway. Everyone going in and out of the fortress goes down a little ramp, through the 4/7 water learning some swimming skill, and back up another little ramp on the other side. The pressure plates keep the depth at 4/7. If invaders threaten, I let them get into the entranceway, then throw the lever. The floodgates snap shut, and the chamber fills up two z-levels and drowns them.
Will this work? Thoughts? I'm a little concerned because I know there are some bugs with pressure plates and water depth, but I can't think of a better way to keep the water at 4/7 depth with a constant inflow from a waterfall.