First year saw the 2x2 staircase walled up and roofed over. The first few years my defence was mainly handled by the one tile wide cage trap corridor (the
^^^). After I had a few skilled squads and had some more of the surface structure built up, I controlled the amount of invaders inside with drawbridges, and had my militia take them on in the corridors. As my militia grew in strength (year ~3-4), I would take on bigger and bigger batches of enemies. Some time after the five year mark I deconstructed part of my staircase wall, enabling my dwarves, caravans and any possible invaders to completely bypass the cage corridor.
For the past years I would station my squads in the lower left microcline room and from there charge all ambushes and small sieges, while trying to bait portions of larger sieges inside. At least one such attempt resulted in my legendaries charging straight outside forcing me to commit all my forces. I haven't been counting but I've lost maybe a dozen or so dwarves in combat (standing army 21 + fortress guard, all melee). I've never lost a high master or better. Another twenty or thirty and you'll have an rough count on my civilian casualties (mostly ambushes while building the surface structure).
There are three weapon traps before the lower drawbridge. Enough to mutilate any stray wild life but will get mangled after the first two ambushers to allow my militia to do most of the killing. There are three wooden spike traps preceding the cage corridor, but they act primarily as a failsafe should my militia fail.
There was no real plan while I built the surface structure. It just sort of evolved from a 4x4 roofed stone hut into what it is today, adding a new bridge or a twist in the corridor every few years. It could certainly use a lot more planning. Maybe next time. The few twists in the corridors are to decrease the threat of (elite) ranged units, a job which they've done. Only one of the drawbridges has recently been outfitted with a 2 z-level drop into spike traps, the rest are merely 1 level drops to prevent enemy traffic.
The reason for microcline is that I ran out of grey stone halfway through and didn't bother digging for more.