Guys, it is worth noting that using a mage to build something is going to be like having a modern day potter make something.
I know several artisans (potters and others.) They cannot, by hand, turn out identical pieces. The human mind just isn't capable of it.
Would having a mage speed things along? Simplify things? Certainly.
There's also knowledge - like the "float glass" method to turn out glass with an even thickness - that the Halkegenians might not have. If they don't, then the Fae can provide that. Before "float glass" was invented glass was often bubbled, cracked, or warped & uneven when it was finished. Cracked glass could be melted down and recycled, but the rest was accepted because they didn't have any other choice.
Do me a favor, if you can - wander around your hometown and look at buildings that are more than a century old. If any of the windows are original components you'll see that the reflections in the glass are distorted. It's because the glass was "less perfect" than we can make in modern times.
The same thing can be said for metallurgy. In modern times we know about alloys that we didn't a hundred years ago. Aluminum was incredibly expensive when humanity first discovered how to extract it from other ores in the 1800's. A few decades later and someone found a new, inexpensive way to separate aluminum from other materials and it became incredibly cheap. There's a reason that many drinks are sold in aluminum cans - the process is just that cheap!
These days aluminum is used for everything from heat sinks to food containers to construction materials. I remember a time, a few decades back, when aluminum racing bikes were all the rage in the U.S. - California specifically - because they were light-weight. (These days they use even lighter materials.)
What else can the Fae show the Tristainians?
.. and when can we see it in story?
Light help us, a few enterprising Fae, a few contacts in Tristain and beyond, and Lord Rute's economic knowledge ..
Lord Rute: You know where you failed, Mr. Villain?
Mr. Villain: Damn you, Rute!
Lord Rute: You wanted to conquer the world.
Mr. Villain: And what about you? I've seen your mercenary armies, your death-squads!
Lord Rute: Death squads?
Yes, of course I have death squads. They couldn't possibly be internal security for Faerie Advance Enterprises. No, no. I don't want to conquer the world. *grins wickedly* I want to
buy it.