I imagine, out in the Badlands, something that from the outside looks like a giant termite mound-inside, it is more like an ornate Canterlot Castle. Everyling has his or her personal home within it.

I've switched my view to a castle where the changeling have a civilization and a monarchy similar to equestria

Well, first the changeling would have to be exposed to something they're really allergic to... um... not that kind of hive?

a large, ornate complex which, like Canterlot, resembles a castle from the outside, but is actually made up of several buildings

A termite mound would make the most sense in the desert, maybe even so large that it is mistaken for a small mountain, thus providing them with extra concealment as it no longer looks like a structure.

I think it depends where they live.

A deep, deep tunnel in the ground that eventually opens up to a colossal, egg-shaped dome plated with the same material that the changelings are made of. I think it's like Canterlot, but all changeling-y.

In my story, the changeling hive is primarily concerned with secrecy. As a result, it's hidden away in the badlands, and mostly underground. The bulk of the hive is in various chambers and tunnels, mostly unadorned, with dried changeling-goop (Basically like wax) often covering surfaces in the lower sections of the hive to keep water from seeping in. The only above-ground portions are the spires (Made of the same wax mixed with rock and debris to form a tough building material), which act as lookout towers, barracks for the soldiers, and defensive structures. They rise up in a box canyon, so one would only be able to see them if one flew directly overhead, and in a rocky area of criss-crossing canyons that would make navigating by land almost impossible. This is how they remain hidden.

Inside the hive is pretty spartan. Other than the work spaces, there are various communal sleeping chambers. They don't need silly pony things like private rooms, or personal space, or beds. Changelings are tough, and practical! They don't need excessive luxury like those soft ponies do.

(...And then infiltrators go into pony society, and find out that maybe such extravagant luxuries as running water, cooked food, and a soft and comfy place to sleep at night isn't necessarily so bad. In moderation, you know...)

781531 I agree with you however i consider that changeling bodies cannot support normal delivery and thus the changeling parents create some sort of "embryo/egg" that grows inside a coccoon with the help of the caretakers and the parents of course. So in a few words what you see as "hive" is only the incubation chambers of their cities and because of them now being chased around they havent had the chance to build a normal city.

My personal view is a vast underground network with a couple of disguised entrances to the surface; thus how they've gone undetected for so long. I imagine that the place would be more than just sticky goo and dirt tunnels and no interior lighting, though. They've probably been borrowing ideas and architecture from other cultures for a long time, so it'd probably be done up fairly nicely on the inside.

I'd think they'd have them in plain sight honestly. Think about it this way: A Changeling can walk right up to you, but would you know about it? nope! So the best place to hide a hive is in plain sight

I always pictured it as a mix of basic masonry and ornate chitin. Sort of similar to Equestrian architecture in terms of proportions and basic shapes, yet clearly not the same. I usually picture it as being in a massive underground cavern, but not actually built into the earth (on the "surface" of the cavern floor).

Something that could be a good inspiration would be old asian architecture - simply imagine the paper walls replaced by translucent green goo walls with the consistency of amber, built upon a support structure that is made by air-drying a mix of goo and rock. Depending on what enzymes they add to the goo, they may even be able to dig tunnels by spitting it at the walls and letting it digest the rock.

For some reason, I always saw the Hives as dark, dank places, tucked away in the most remote of areas, the Badlands, deep in the Everfree, places like that. It'd be like an ant-hill. Mostly underground, filled with dozens, if not hundreds, of chambers.... I always imagined a sort of caste system dominating the changeling life.