What makes me wonder is how the smoke from the attack on the 1st wall can be seen clearly from the city? I don't know how far the 1st wall is from the 2nd wall, because it was never mentioned. But in chapter 46 it was mentioned that the distance from the 2nd wall (which separates the agricultural zone from the demon forest) to the 3rd wall (which is the city boundary) is 30 km. So the distance from the 1st wall to the city wall is more than 30 km.
It looks like they're looking at the smoke from inside of a building, maybe the second or third floor? That would increase the range at which they could see the smoke.
I did some quick googling, and I've seen people saying 20 or 30 miles, depending on weather conditions, the size of the smoke cloud, and what's being burned, but take that with a grain of salt.
What I find really interesting is how the plot "demands" that suddenly the main gate, shown with some impressive artillery earlier, simply takes a helpless pounding.
That's not even considering the presence of those drones, or the fact that there's an actual fully automated space warship in orbit capable of orbital bombardment.
Yet they're being pummelled by some cobbled-together steampunk toys without being able to do anything but "close the gates"?
Now that's ....sloppy writing...
Chapter 46, Page 8. The gate of the first wall that's shown has no defensive guns. It's just a brick wall with a gate that can drop down.
And on the next page, it shows the pylons which are sonic anti-monster repellant devices, so that explains why there are no monsters attacking the enemy army. The characters mentioned how, even though they're passing through the infamous Great Demon Forest with a caravan of civilians, they haven't detected the presence of any monsters.
So there do appear to be explanations for why the enemy army wasn't attacked by monsters, and why they had such an easy time setting up siege weapons to destroy the outermost wall.
The weapon emplacements you asked about are on the second and third walls, which are shown on Pages 10 and 16.
Page 10 also shows that the second gate is the first one that Alan had to be scanned at for entry.
Edit: As for the drones, they were keeping an eye on the army, so they probably had some reason not to intercept them too early. And the ship may be automated and in orbit, but it's heavily damaged. I don't have the chapter and page, but I do remember that it was said that the hangar was destroyed, and that the rest of the ship was still heavily damaged. While it can repair itself to be able to fight a Bug fleet (Chapter 20, Page 17: perhaps a scout fleet, since Iris said BG-I Type cruisers could be handled, but a single BG-X Type cruiser would be unbeatable), the assumption was that the repairs would take 500 year.
So any weapons the ship has remaining might still be too damaged to operate.