I believe that the technology portrayed in the series is a mishmash between medieval weapons, like the scavenger trebuchet used to bring down Winter in Winter Turning, and more steampunk items, like the "mysterious devices" mentioned in Darkstalker, and the technology the HiveWings are developing on Pantala. 

After reading most of the series (I still haven't read The Lost Continent yet, plz don't kill me), I've concluded that an appropriate model to base these deeper studies off of is the Viking society in How To Train Your Dragon. Of course, the architecture of the dragons' villages and cities varies depending on the tribe, so this assertion only applies to the tech.

The Vikings in HTTYD have primitive technology, like spears, swords and other types of handheld melee weapons. But at the same time, they have flight suits, diving bells, crossbows, projectors, ballistae, grappling launchers, and flaming swords, to name just a few of Hiccup's inventions. This style of technology is primitive and yet it fulfills the role of modern tech in a lot of areas. Being a sci-fi junkie who has been exposed to genres like steampunk, atompunk, dieselpunk, and whatever punk you can pick up at the local rock concert, I have come up with a name for this new tech style: Vi-Punk, or primitive-punk. (That name isn't that great plz help me with a better name)

My theory is, the same is true for the technology in Wings of Fire. It's comparably primitive tech, but it's quite advanced for its crudeness.

The dragons, much like those curious scavengers they are known to eat, aren't stupid. They know how use the materials from around them, like iron, wood, and other metals, so they're not completely primitive; they must have some form of metallurgy and blacksmithing, because of the armor and weapons the dragons use. 

Their weapons are medieval or ancient in style, with more advanced materials like steel being more experimental. Siege weapons like ballistae are also used. 

Running water and plumbing will be covered in question #2, while the various materials the dragons use will be covered in question #7.

Also, we know that they have whales in the world of WoF, so it makes sense that the SeaWings could have discovered whale oil. Plus, they mention lamps and lanterns and torches. What are those lit up by? So, as far as that kind of stuff goes, it's pretty likely that oil works as a light source inside the context of Wings of Fire. 

*insert Vinny meme here about BIG BOOM*

Explosives do exist in the form of dragonflame cactus, which grows in the Sky Kingdom and was used to create a bomb in Moon Rising. So we know that there's more advanced technology than just digging by hand in gold mines (more on this in question #8). Plus, this would make a very intense weapon.

Now, this is where my personal canon gets a chance to shine. In my fanfic series, Wings of Fire: The Misfits (go check out my profile where the first book is already up and in progress of being made), there is a large trade syndicate called the Barterguild. In addition to commanding the commerce of Pyrrhia, the Barterguild also runs tons of scientific laboratories and educates dragonets in a similar manner to Jade Mountain Academy (more on that in question #19). If any dragons know about new technologies and discoveries, it would be the Barterguild. They know how to use the tech available to them and come up with new ways they would be used. A dragonflame cactus could either go to help miners dig or to kill other dragons on the battlefield.

So, a definitive answer to this first perturbing question is definitely "A mishmash of primitive and advanced technology, fitting with a wilder, less modern world."

Stay tuned for more perturbing questions and intelligent answers!