That is quite the elaborate set up for (remote?) snowy mountains. Good thing it was a white dragon so it was actually stupid enough to be lured into such a position.
That's how you fight superior foe. Ambush tactics. Not so remote, it was part of dwarven kingdom, one of their supply routes. Dragon wasn't stupid. It knew dwarfs use those mountain roads to haul goods, which was primo place for easy fly by plundering of livestock and food supply. So party tracked dragon movements, patterns, with help from caravan people and route wardens. Extensive scouting and research about foe was done, so was in choosing right time and spot for setting ambush point.
And "level 4" automatically negates qualifying as "normal humans" as per the OP, as does the wizard casting web since no spells was part of it as well
Lv 4 in 2ed AD&D. Our party fighter had 15HP, ranger 18, thief had 10, wizard was at 8. Don't know if you played it, but you rolled for HP at every level, even first. So fighter with 1 hp at lv1 was also possible. Also, stats were 3d6, arrange as you wish, so good luck getting that extra 15 needed to put into CON for extra 1 hp per level. Normal human would definitely fall into lv 3-4 ad&d character. As in, 1-2 good hits from sword and you are dead.
Yes, numbers can be key in general, but that scenario screams to me of a DM allowing you to "get away with it". I mean, "steel cables anchored into mountains", "stone pillboxes"? Quite a bit of a stretch IMO.
No. It was just few sessions of extensive prep work to set things in place, formulate plan, gather equipment, resources etc. Overlapping fields of fire, negating one big dragons advantage (flight), surprise attack with artillery, and of course, right spot. Pillboxes where just basic round low stone buildings with slits for crossbows, same type farmers build across our islands for hundreds of years ( they are built across Mediterranean region,specially islands) as shelter from storms. Built couple with my grandpa as a kid in our olive gardens. Fighting dragon in narrow gorge is like infantry fighting tanks in urban environment, same tactical principles, modified for fantasy.
As i said, 2ed was much more realistic, specially low levels. Low hp, low to hit, you needed way higher stats for bonuses (at least 15, sometimes even 16). So normal humans have same chance as PCs to survive low level adventures. They just have to play it very smart and don't try to act a hero.