I'm going completely overboard in my defenses... I already have a dwarven atom smasher in a hallway that has about 20 straight traps in a row down a 5-tile wide entryway, plus I'm working on an extended set of drawbridges that you have to enter sideways, because the far one flings people into a pit filled with water and aquatic pets. (Now I just need to start breeding cave crocs...)
However, even this is not enough for me. (Note: no seige has survived my first three layers of traps so far. I really should have upgraded to orcs.) I am now designing a plan to make anyone trying to enter my fort run across a long, exposed walkway, but to also require they go UP a long, corkscrew-like platform that twists around one of two towers, then crosses over to the other tower, and corkscrews back downwards before finally reaching my enterance. Inside these corkscrew towers, there will be fortifications so that my marksdwarves can fire at them the whole time, while the entire length of this bridge will be one GIANT string of spike traps.
It will keep me occupied for years to come, and hopefully, it will take about a month for caravans to even traverse the terrain. (Note: I'm currently working on a "screw the sun" project, where I will never have to go back up topside again, and entirely subsist on the forests of tower caps and greenhouse trees. That way, a massive, wasteful obstacle course won't really hurt my overall efficiency.)