lol aliens gave celest levitating buildings now too?

We are able to build earthquake resistant buildings. Lusternians have magic and have been working on it longer than we have.

Earthquake-resistant buildings don't survive giant chasms opening up beneath them-- it's something that's not really worth thinking about too much in-depth, though. It's just not going to happen.

Wait...we have a tank?

Well, maybe not a tank, but I recall seeing something that looks sorta like one being built in a room decription.

The Engines of War.
Seemingly endless rows of workers fill this room, and heat ripples can be seen
rising to the roof, disappearing into the thick cloud of smoke that hangs there.
There is an overwhelming sense of noise here, metal banging against metal, gears
grinding and the high-pitched squeal of iron being carved into sections. The
seemingly disordered rows become clear to you as assembly lines, each dull,
repetitive task contributing to the greater finished product, which are visible
at the very end of the room, being finished and polished. You see strange,
wicked-looking ballistae, and massive catapults, adorned by murderous spikes and
plated with iron and steel, all looming over veritable mountains of gleaming
swords, halberds, battleaxes, flails, sleek javelins and even huge warhammers.
If the air seemed thick and polluted outside, here it is nigh impossible to
breathe, and some of the weaker workers even faint from time to time, though
they are soon forced roughly back into line by glowering supervisors, wearing
thick half-masks to cover their mouths, this protection is not afforded to the
workers themselves, of course.

That room? It's not a tank but a bunch of ballistae and catapults.

As for geomancers taking down a building, I assumed that it would just be too much. It would be fairly large chasm, and they would be moving the earth. A lot of earth. If the whole guild got together It could happen, but it would be slightly hard anf a long and tiring process.

umm, armour only protects against cutting and blunt. It doesn't protect against elemental magics.

I always thought that gunpowder was a stroke of luck. I mean, in medieval times, before modern chemistry - Did someone really go like, "I get it! It's saltpetre, sulphur and coal!" and then proceed to make black powder?

china had gunpowder in ancient times..

In real life, in went something like this:

Alchemist: Okay, lets mix this, this and that together and see if it turns into gold.
Alchemist: Not gold. Lets try heating the results and see if that makes it gold.
BOOOOM!
Alchemist: Amazing! Lets show it off to everyone else.


Realisticly, It would provide some protection.

Not exactly "ancient times", but they went from smoke bombs to rockets in the latter half of the middle ages. Guns and flamethrowers in some parts.

how?

Metal armour, gets very cold or very hot, and would hinder the person wearing it.

Magic also can go past armour, since it is you know, magic.

Except what the magic does is create a 'bolt' of energy, whether that be fire, water, or pure magical energy. Armor acts as a barriar to most of it

Except we know that it doesn't. Warriors train themselves through athletics to deal with magic damage.