1) How did the halflings notice the gnolls approaching in the dark to the point where they are already hidden in a completely fortified location?
They didn't need to. They went to bed independently of the gnolls. In their homes, under the ground. Which are fortified. Some of then are up reading or arguing with people on the internet.
2) What do you mean "extremely solid doors"? Where is this coming from? Even if it is a really strong wooden door, it is likely a DC 15. Gnolls have a +2 strength and the help action. Breaking down the door isn't that hard.
Reinforced doors. And once broken then what? The gnolls start crawling? Because the ceilings are about 3'6" high.
3) Why would you burn the ground? You shoot flaming arrows through the windows of the house, and set fire to the inside of it.
Let's set the earth on fire. Great plan there.
4) Wait, now the doors are extremely solid, reinforced, and concealed?
If living in hostile country and expecting raiders
of course. They're solid and reinforced. You think they wouldn't reinforce their doors if expecting
gnolls?
5) Yeah, I covered squeezing. A gnoll can fit into a small space, like that 3 ft ceiling you mentioned.
Do yourself a favour. Try crawling through 3' high tunnels and then fight. Especially when the layout was designed so you can be poked in the side with spears. Gnolls, to move through halfling burrows aren't just squeezing but simultaneously squeezing and crawling (which means that they count as not just squeezing but also prone).
Sure the game mechanics allow it - but I want you to look at the physics. How are they carrying their spears while crawling? This
definitely counts as a fortified position - which gnolls explicitly do not attack.
They have disadvantage on attacks and advantage on being hit. "The first gnoll to be killed" assumes that a gnoll in melee killing two halflings a round
If a crawling gnoll is killing two halflings per round then we have extremely stupid halflings.
You seemed to have just assumed that halfling homes are impossible to breach, with their glass windows and wooden doors.
Yay. Because halflings have large glass windows in gnoll country. You have assumed extremely stupid halflings.
And that somehow the squeezing rules make a gnoll helpless. That isn't how that would work.
It's not the rules, it's the physics. And no they aren't helpless - just comparatively easy targets. Crawling and vulnerable to being stabbed in the backside. And because under the mechanics they are both crawling and squeezing they move slooowly.
You need to review the Squeezing rules. Halfling homes aren't an inherent defense. And then humans should also get slings and default to having slings available.
Yes, because humans have a noted affinity for slings across multiple editions. And are explicitly community minded. Oh wait.
It is the best way to defend themselves and it isn't like they wouldn't want to do that.
Please stop with the inventions. Slings are not
the best way to defend yourself. Their main benefit is that they are
easy to carry from day to day. So if you're a relatively egalitarian community with everyone pitching in it's a good way to ensure everyone can. If you're a more formal race that likes hierarchies you're more likely to go for militias.