Right now I have a series of 3 drawbridges, the innermost one separates the underground part of my fort from the above ground part, the middle one separates my walled courtyard from the trapped entrance, and the third one separates my trapped entrance from the outside world.
Siege comes, usually none of my dwarves are outside so I can just pull the innermost lever (my soldiers train outside). The remaining two bridges are open, I take stock of the invaders. If there are any building destroyers, I close the middle gate, leaving only the traps.
The non-building destroyers stay still, as there's nothing for them to attack, while any building destroyers beeline toward a set of statues surrounded with cage traps. Outer bridge raises, middle one lowers, anything that managed to avoid the traps gets cut down. Inner bridge raises, cage traps are reset, civilians go back inside, inner bridge raises, outer bridge lowers, allowing the rest of the siege access to my soldiers.
Many are caught in the cage traps, the outer two bridges can be raised and lowered to control the flow of goblins and allow for more trap resetting. The goblins not caught in the traps are cut down by my danger-room trained, steel-clad military. (I've seen my 10 man axedwarf squad take out the remainder of a siege consisting of at least 6 squads of goblins and a squad of trolls).
Cleanup ensues, caged prisoners are brought to the base of my sacrifice tower. Four at a time, their cages are built at the top and their inhabitants pitted down 10 z-levels onto an upright spike. After this is done for each prisoner access to the drop room is opened and their droppings can be cleaned up in time for the next siege.