In Tinfound we've just finished our main defences.
The upper entrance is defended by a "dodge this" spiral which is sealed by retracting bridges when not required:
Example top view (this is the highest z of the spiral, incorporating the entrance from outside):
www
wwwwwwd d dwwww
wf f f f f f wf f
wf b b b b b b f f f f
wf b b b b b b f f f f
wf b b b b b b f f f f
wf b b b b b b f f f f
wf f f f f f fw
wwwwwwwwwwwwwww
Where w = wall, f= floor, b = retracting bridge, d = down stairs.
So when the bridges are in place it is a simple journey, but when they are open, anyone who comes through has to traverse the narrow spiral. Top z is bristling with weapon traps, the second z has a bunch of chained wardogs (plus puppies who will likely go into freefall when the bridges open), the third has nothing yet but probably more and nastier weapon traps, and the way out.
Oh, and anyone who dodges the wrong way ends up in a pit full of menacing steel spikes and giant desert scorpions. Good luck!
Our lower level is guarded by a slightly different arrangement consisting of a "toilet trap". Normally it is a regular corridor. But when the emergency lever is pulled, a bridge retracts leaving the only way across as a narrow ledge, with a lot of pressurised water on one side (aquifers are damn useful) and a 10z drop into a pit with a bronze colossus in it, on the other. This side isn't fully functional yet, as the water cistern digging has been extended by the discovery of unexpected coal reserves, but it soon will be.
Also, it's really annoying that dwarves suspend construction when the builder has lost their baby. That delayed my second aquifer breach so many times, that in the end when Urist McBaby decided to play in the water again, I just turned off the drain pumps...