My favorite nostalgic moment was the first time piercing the pit of HFS. I hadn't the benefit of the wiki back then, just a few videos from Captain Duck. Seeing those dwarf corpses really put me on edge. Immense fun ensued.
HFS in 40d was a lot harder to kill, especially if you encountered a certain type of them...
Back then, Spirits of Fire were pretty much dwarven death personnified. You know how fire-breathing creatures are somewhat dangerous but can be bested by carrying a shield? Well imagine taking the fury of an entire fiery wave of death, compacting it in a single ball of fire, and then hurling it at insane speed at your steel-clad champion, incinerating him instantly. And there are about 15 to 20 demons that just spawned that can shoot fireballs. And they're in ambush mode until it's too late. And you burn to death when you got close to them. Congratulations, you just opened an adamantine-lined can of Spirits of Fire, also known as !!whup-ass!!.
Coming back as an adventurer and having your entire upper body reduced to ash by one of those fireballs can be quite jarring, since it comes out of nowhere.
I also have fond memories of early fortress defense experiments. I always found the idea of ballista batteries very appealing, since it was basically launching an entire tree with a pointed tip at someone. Let's just say that after flooding a good part of the fortress trying to build a water moat to funnel invaders in front of the ballistas, and then ending up skewering more dwarves than goblins, I don't really use ballistas much anymore.
Another thing is that I usually thought of sieges more like an invitation to do battle in an open field. Seemed more epic that way. I had absolutely no other kind of defenses other than maybe a few weapon traps scattered around along with chained war dogs, and no drawbridges or moats around the fort entrance proper. Also my farm was outside right next to the trade depot. And it had a hallway going deeper into the fortress.
The goblins REALLY liked my dining room though, because when I came back as an adventurer, they were all hanging out in there.