TLDR: What would an internment camp look like, if it were designed for magical beings in a high medieval society (with no gunpowder, but fantastic metallurgy)?
I am building a world wherein there is limited magic. However, in its past there were magical beings of all shapes and sizes from sprites and fairies, to warlocks, witches and wizards, oracles and shapeshifters, even elves. This world is in a period with tech equivalent to what we would call the high medieval period. However, they do not have gunpowder; but they do have fantastic metallurgy allowing them to make mass produced steel using a technique similar to the Bessemer Converter.
Definition of Magic
Humans --- The vast majority of magical humans are peaceful, and an average one would be able to control inanimate objects through thought to do basic tasks (e.g., sweep a floor, or sharpen a knife, etc.). However, some have taken to pursuing how to manipulate their gifts and are quite powerful resulting in skills that have taken these basic ones described above and being able to utilize them to create massive amounts of energy (firecasting, directing lightening from their hands, shapeshifting, and the ability to take control of animate objects through telekinesis which in extreme cases would include people). Conversely, you would have some of these people who did hone their skills, and get quite powerful on the opposite end of the spectrum and use their magic for goods instead of bad (this could include healing enchantments, the ability to shield others from telekinesis or other means of countering the vast powers of their more chaos persuaded peers). Additionally, you would have basic conjurers of all kinds (some of which wouldn't even be magical, but just roped in and consequently imprisoned due to the quality of their tricks), alchemists (both medicinal and magical), fortune seers, and more.
For frame of reference only about 3-4% of the human population is magical, and of that maybe 25% of them chose to pursue and hone it to something "bigger" (so roughly 1% of the population overall). This would put the magical human population at a staggering approximate target population of 350,000 humans. Prior to the cull magical and non-magical people typically lived together in an almost symbiotic relationship, there was even intermarrying of the two groups (while uncommon it was not socially frowned upon).
Non-humans --- For the lesser important (to the story) we would see creatures/species very similar to what is readily established in popular literature the fairies and sprites would be small and at times troublesome, with limited magic. While the elves would have a different knowledge and more in depth than humans in terms of herbology, they would also live longer. However, therein lies the problem due to their difference in biology their fertility rates are lower and despite being skilled warriors, wise and living longer they have a smaller population to that of humans (out numbered about 15:1). Some elves, like some humans, are also magical and face the same constraints/potential as humans (in essence some powerful, some not, some honed their skills, some didn't). Goblins were long extinct, due to atrocities committed by the elves in ages past. As a result there are some of their artifacts around but nothing left of them, their culture - just ruins, old weapons, etc. Dwarves do not exist in this world. Trolls, ogres and giants do - but prior to the cull their numbers had declined (no one knows why) to the point that they are almost stuff of legend due to their rarity.
Using that same methodology this puts the population of the magical elves at roughly 25,000. Now, due to a different social construct it is far less common for them to mix magical with non-magical in terms of relationships and child rearing. They will live and work in the same communities but would run in different social circles as a result of different sub-cultures developed over the course of their society's development.
Due to some geopolitical destabilization in the region caused by Lord Farquaad who has descended into a schizophrenic-like induced paranoia, which he attributes to a curse or bad omen, he decided to round all he could up (some the logistics were too difficult like the very few sea creatures he deemed magical), put them in camps and ... you get the picture. It should be noted that there were some non-magical people/creatures rounded up as well if they gave the illusion that they had powers or simply looked like they should have them (as described above). Due to his paranoia, let's assume money is no object and that if needs dictated it he was willing to build multiple facilities across his lands.
The time in which the various after effects of the "magical cull" are where my story takes place. However, I am stumped - what would such a facility/fortress/prison (call it what you will) look like? What weapons or defensive structures would be left behind 200-300 years later? I know that the stone ramparts, walls, and some of the metal products would be intact(ish) just from looking at the ruins in Europe and China of castles, walls and fortresses. But would would this place look like, in its prime (so that I can extrapolate it's state and appearance for the timeline of my story).
I have added more details, broken down the magical creatures by human/non-human and tried to better frame the question. I have also added some more information about the Lord Farquaad's motivations and some of the social dynamics of the time. I believe this answers the questions raised in the comments.