1 Quibble: the Romans, at least, were iron-age (though pre-industrial.) Based on their military tactics they were probably early medieval, post-Gothic-invasion.

2

My assumption is that they were bronze age when they came through the gate and learned about iron and steel afterwards.

Otherwise I can't explain why, after maybe a thousand years, they haven't advanced more. Mimoza told us that humans were the most recent arrivals, and we know that now there are millions of them. Enough so that 120,000 men were killed fighting the JSDF at Arnus.

Those were all regular army from the Empire and surrounding kingdoms. If collectively they could maintain an army that large, the population behind it that furnishes the men and money and supplies must be vastly larger.

3 I'd expect a variation on that, where the Japanese scientists are cooperating with the mages. I mean, let's face it, your average physicist, given incontrovertible evidence of magic, would sacrifice any random body part to learn how it works.

Meanwhile the mages are faced with evidence that knowledge of Japanese physics enables to to do much more powerful magic.

They'd both crawl across broken glass to keep in contact...

4 It's pretty easy to understand why they didn't advance more - magic exists.

Take a society where only a fraction of the population is even exposed to reading and education. From those, take a significant percentage and tell them "you should study magic, which will let you throw fireballs and call lightning and fly". The result: damned few people with the smarts to do engineering and the inclination to bother.

There's plenty of research going on, but mages do magic research and get solutions for mages. Most of them aren't going to spend time on the kind of things that would benefit mundane society. (For example, no guns, not even the concept of guns, though even a fairly crappy mage could design one...)

5

 

In the Roman Empire there were two kinds of engineers. Military engineers worked for the Army and built field works and fortresses and siege engines of various kinds.

And Civil Engineers built buildings, roads, sewer systems, bridges, and aquaducts. That's where the name "civil engineer" comes from; it's a translation of the Latin term.

Neither concerned themselves much with metallurgy, but they both needed good metals, for tools (for instance). Plus the Military engineers (and the Army in general) needed metals for weapons.

Among other advances, Civil engineers discovered and perfected concrete. They also learned a lot about mathematics (despite the horrible impediment of Roman Numerals) and were able to design structures which still stand, 1800 or more years later.

One of the most amazing is Pont du Gard, which was part of an aquaduct which supplied water to Nemausus (modern Nimes). 20 or 30 years ago I saw an article about it in Scientific American, where some modern civil engineers analyzed the design using modern tools and knowledge. The fact that it still stood more than 1900 years after it was built was remarkable, and the usual assumption until then was that the Roman Civil Engineers had massively overdesigned it.

But the modern analysis found that in fact it had about the same kind of overdesign that modern Civil Engineers tend to use.

The Empire has a very advanced civil engineering technology; we can tell that from some of the scene shots of the Capital and from indoor images at various points of one or another palace. They had glass windows, which was a very advanced technology. And their metals were generally excellent. Armor for horses is not a primitive technology.

None of these things are affected by magic one way or another. Magic can be of use during construction, for instance to clear away brush and rubble, but the actually design and building would have to be done the old fashioned way, by strong hands and backs.

I don't think magic is the answer to why they hadn't advanced further. I think the answer is "gods". Science in our timeline finally begins to bloom and expand once a mechanistic view of the universe becomes more common and acceptable (i.e. doesn't get you burned at the stake). With the assumption that the universe is findamentally mechanistic, ruled by immutable laws, you begin to make progress.

That isn't possible in the Empire's universe because there really are gods; they're not mythical. And they select small number of humans and give them super powers and amazing weapons and task them with maintaining control and preventing things from going out of control. Rory talks about this in the manga and hints that one thing the Apostles have been doing is to stamp out technologies which the Gods think might be disruptive. Get too creative and Rory or someone like her shows up and slaughters you.

6 So, basically this world is the gods' play room. They bring in some new toys once in a while to mix it up, and even let the occasional toy ascend to their status, but you don't let the leggos advance to the point where they might challenge you.

The Romans did some amazing things, but there's a certain degree of survivor bias here; Any shoddy work they did is long gone. You'll see the same thing when house hunting; If you see a 50 year old house, or better a 100 year old one, you'll be impressed with how well built it is. It's easy to forget that it being well built is the only reason you got to see it, and think they built them all that way back then.

7

The biggest problem with immortality is boredom. You might be right that the Gods open the GATE and introduce new races every once in a while just for novelty value.

8 Only problem I have with that is, well... if you've been trying to repress progress and disruption for millennia, then opening a portal to -Japan- ain't consistent with prior policy. (Of course that policy doesn't have to be universal. You can have gods trying to stimulate progress, or simple amusement, and other gods running around trying to stamp it out...)

Which is also interesting, because Rory emphatically doesn't stamp it out. On the other hand, she does not characterize Emroy as a "good" god and emphatically not as a "nice" one...

Also, I'm not sure that I buy the "there are gods so they don't believe in science". That's not consistent with the mages that we see, who DO believe in science! They're aware that there are "true principles" by which the world operates, and specifically generate magical effects by warping those principles via magic, right? Lelei immediately recognizes that by refining her understanding of the "true principles", i.e. physics, she can make her magic more effective. That's not the obvious conclusion if you don't already have at least the concept of science; if you chalk it all up to understanding the will of god(s) then how does physics help with that?

At the same time, I can see the mages believing this (because it's fundamental to their magic) but not sharing that with the general population (hey, mysteries of the art.)

9

It may be that when the gods decide to open the gate again, they don't know what they'll get, what they'll connect to.

And just to clear things up a bit, here's the part of the manga that talks about this:

It isn't so much that the gods are trying to stop progress; it's more like bonsai where they're trying to control the growth so as to maintain a pleasing result.

10

Of course, "pleasing" is in the eye of the beholder and it's entirely possible that the gods themselves disagree with one another about what is pleasing.

It's also provable that "pleasing" doesn't necessarily mean "peaceful". Emroy, in particular, is pleased by battles since he is the god of war, death, crime, and insanity.

The hands of the gods are the twelve apostles who roam the planet doing whatever they do, presumably things that satisfy their respective patron gods. And it would seem that each patron god can communicate to their own apostle. So Rory has hooked up with the JSDF, and I assume that if Emroy didn't approve of that Rory would know it soon enough. Clearly Emroy doesn't disapprove of what Rory has been doing with Itami.

I get the impression that Emroy and Hardy are not... shall we say... in sync. Hardy's apostle Giselle woke the dragon and bred it to create two more, whereas Emroy's apostle Rory helped kill all three of them.

So the situation with the deities is obviously complex.

But we can't discount simple boredom as a motivating factor. The arrival of the Japanese is the single most interesting and complicating event of the last thousand years, possibly even longer. Irrespective of the consequences their technology may bring in the long run, for gods who are bored out of their minds the arrival of the Japanese has been great stuff.

11 Russian rule of thumb was that a population of 1,000,000 is necessary to raise a wartime division. Squeezing anything beyond that is and the rear line supply will nosedive.

12

I believe that number, though that's for a 20th Century division including hundreds of trucks at at least dozens of tanks and hundreds of tons of munitions and fuel and other supplies. For Roman-era legions, the ratio is probably a bit lower; the limiting factors are going to be tax revenue and just coming up with that many men without bleeding the entire population dry and causing a reproductive collapse. (I'm thinking of something like what nearly happened in the UK following WWI.)

In the case of the Special Territory, it's even more than the 120,000 that were killed in two battles at Arnus. There's also all the guy the Empire lost in Tokyo, killed or captured, and even with that, the Empire has been able to reconstitute its army since the battles at Arnus. So the human population alone must be upwards of twenty million. Possibly way beyond that.

13 By way of comparison, the US in WWII had a population of 170 million and put 8 million men under arms in all branches.  This was with (almost) full mechanization, however.  There were 100 Army divisions (plus six Marine--and a few numbers were skipped); however some of those were training or cadre divisions.
Of course, divisions weren't exactly 10,000 men either; old "square" divisions had about 12-13,000, whereas tank divisions were smaller. 

14 Could Hardy be behind the opening of the Gate this time?  He's been playing a lot of games behind the scenes, it seems.

15

Rory has dropped a snide comment that suggests that she thinks it was Hardy who did it. But she might be wrong. There's really too much we don't know about it for us to be speculating at that level. (Of course, we can speculate about anything, but our chance of being right is no better than random.)

Also, Hardy is female. That was definitely confirmed in the encounter with Giselle.