Jun 8, 2013
Make a back entrance...
…with two retractable bridges leading to it. Would-be invaders cross one, step on a set of pressure plates in the middle. Both retract and leave them stranded in the middle. Pressurized water from the side to brush them into one of the following: 
 1. A pit 
2. A shaft to a cavern layer.
 3. A shaft to the magma sea.
 4. A set of cage traps (for later entertainment or training)
 5. A shaft whose base is lined with upright spears, or, for bonus dwarfiness:
 5a. The bottom of the speared shaft is your dining hall. Waterfall optional.