My current fortress has a gatehouse with a 2 tile moat around it, filled by channel from the nearby river. In the middle of the gatehouse is a 3-tile wide ramp that goes down 3 levels and goes about 30 tiles across a drawbridge that spans a chasm. The main fort is on the other side of the channel, so any sieges need to cross the outer drawbridge/moat, cross the 30 tile long hallway that has retractable spikes on a switch (I use regular traps very sparingly - more as alarms than kill instruments) then cross the inner drawbridge/chasm. The depot is just on the safe side of the chasm.
One of my favorite designs is a 3-wide ramp that spirals slowly down from the surface to about 8-10 levels down. In the center of the spiral is a 9x9 room with fortifications from top to bottom. Archers can stand in the room, shoot out in 3 directions at attackers, and then go down one level and do it again, etc. Traders take a while to get down there, but all the turns make it really difficult for gobbo archers to do much harm. For extra fun, you can install a ballista at the end of each run behind a fortification. With 8 z-levels to cover, that's 16 ballistas they need to survive. There's a tiny alternate entrance to the top of the fort with a drawbridge for the start and hauling in from outside.
My current fort design is comprised of 9x9 rooms in a 5x5 grid and I just drill down as far as needed - usually 16 levels or so. The most efficient path design is a cube with ample staircases (I reserve 4 of the 25 rooms per floor to put in 4 staircases each as well as food/booze/tables/chairs so nobody goes far to eat. That's on the safe side of the chasm. Under the gatehouse is the barracks, weapons stores, archery ranges, jail, animal training. The gatehouse connects to the main fort about 8 levels down.