So, lore-wise, here's what I can put together regarding Ruidus specifically in comparison to other sealed things:

The cage around Ruidus appears to be very similar to the Divine Gate, which is repeatedly described as a lattice. The Divine Gate very specifically only keeps the gods from reaching the Material Plane: travel to and from other planes is possible for anyone else with the spellcasting ability or with access to a portal.

Ruidus is on the Material Plane, a statement I have bolded for reasons of "if you know, you know"; though it has been seen in the Feywild as of late.

Ruidus is also implied to be a prison for two now-unknown deities who had been sealed away and forgotten prior to the height of the Age of Arcanum. It is also indicated to have not existed at the time of the Founding, though it was around by the time of the Calamity. Call of the Netherdeep uses some very deft language to sidestep whether it's truly canonically an alien, dark power that "seep[ed] through the fabric of reality" or if that's just a legend, but does note that Ruidus does have an influence on the fates of those it flares upon. There are enchantments intended to make scrying on Ruidus or discerning its true nature difficult, which is what the Veilscatter Scope bypasses.

Here's where it gets interesting. The Divine Gate is permeable by the avatars of deities and by their favor (ie, Melora can speak to and grant powers to Caduceus and Fjord even though she can't physically manifest on the Material Plane). We know that whatever barriers existed between the Betrayer Gods and the Material Plane prior to them being broken at the onset of the Calamity could not be breached in this way, ie, Betrayer Gods could not grant powers to their worshipers. If Ruidus is surrounded by something similar to the Divine Gate, in theory, that which is on it is sealed off but can grant powers. Which makes sense, because, you know, Imogen.

Imogen notes that the cage around Ruidus distinctly does not look like ley lines. It's worth noting that one of the other barriers we know of, the long-since blighted Tree of Names, was tied to the ley lines. It's also worth noting the Divine Gate is divinely crafted and the Tree of Names was at least implied to be created by the mortal Gau Drashari druidic order. That said, Ruidus itself seems to have ties to the ley lines and planes given that the celestial solstices (of which apogee solstices are a subset) involve the alignment of celestial bodies on the Material Plane leading to a thinning of planar boundaries and potential shifts in ley lines. We also know ley lines can cross planes, although we don't know if this occurs naturally vs. only when wizards get cocky.

The city meanwhile can't really be explained by anything here, so...that's new, and I would presume (though I could be wrong) that Otohan and "guy who is probably Ludinus but could be another elf" are not physically on Ruidus, so the exact nature of Imogen's dreams is still in question. It's also not clear why Imogen has seen several recently departed people, notably Bertrand and the Lumas Twins, in her visions. I'd love to know if there's a third (fourth?), even more secret death god on the moon, which is a wild thing to say.