Regarding Riverrun-2, would a Lord Paramount actually have the power to just take a chunk of land like that from an existing lord? And would he need permission (from the King?) to build a castle and start charging a toll?

Well, it’s not entirely clear who the land at the confluence of the RedFork and the Blue Fork belongs to. There aren’t a lot of noble houses listed in that area – there’s the Mallisters up on the coast, but Oldstones does not have a ruling house (hence the whole business with Jenny), there’s no known ruling house of Fairmarket, and the only other named house from that region are the Blackwoods, and they seem to be center more to the west (directly north of Riverrun). If the land is not currently occupied by a ruling house, than the law of escheat says it reverts back to the liege lord.

As to whether you need permission to build a castle and start charging a toll, there isn’t explicit text either way. However, if King Daeron II had the authority to give Daemon Blackfyre the right to build a castle in the Crownlands, my guess would be that the Lord Paramount of the Riverlands has the authority to grant a license to build a castle on its own lands. And the same logic would likely hold for tolls, given that it wouldn’t be interfering with a royal highway and if a lesser Houses like the Freys can charge tolls for bridges I would imagine the same would apply for their liege lords.

How long would it take to build a castle like Riverrun 2 and what would you call it?

Years if not decades, based on historical examples. As for a name, @goodqueenaly suggests “The Tines,” since the tine is the sharp point of the Trident and this castle would be a strongpoint on the Trident. Also, it’s a nice echoing of “The Twins,” another bridge-castle of the Riverlands.

How exactly does slighting a castle work?

To quote Richard Nevell, an archaelogist who did his thesis on the subject:

“One of the key challenges in identifying slighting is that the methods used to slight a castle were the same used while attacking it. For example, mining or undermining was used in siege warfare to bring down outer walls. But you could use the same approach to demolish a castle. This can be seen at Bungay Castle (Suffolk) where excavations in the 1930s identified an unfinished mine gallery underneath the great tower. Documentary sources show that the King had ordered its destruction, but it was evidently reprieved at a late stage.”

Given that the objective is to render the structure incapable of being used as a fortification, the focus would be on those aspects of a castle that give it defensive advantages:

the outer walls, which allow a small garrison to hold off the enemy from a safe distance.
the towers, which provide further height and protection compared to relatively open ramparts.
gatehouses, which protect the most important entryways with portcullises, arrow-loops, machiolations, and murder holes.

Depending on how emphatic the slighter wanted to be, they could leave the rest of the structure intact and allow it to continue to exist as a

château

(a non-fortified stately home), or they could render the interior unlivable and let the ruins be subsumed back into nature.

Generally, slighting would be accomplished by mining at the base of the structure in question and then burning down the props of the tunnel (i.e, undermining). Later on, the process was sped up enormously with the use of gunpowder, which could be straighforwardly detonated in mere minutes. For the destruction of interior elements, pulling down roofs and then firing the interior was usually sufficient.

Hi! Long time fan. Quick question, what’s the difference between a castle and a citadel?

Hi! Glad you’ve been enjoying the work.

Good question!

Citadel is a word that’s used in a slightly confusing fashion when it comes to fortification. For example, citadel can mean a fortress attached to a city (whereas a castle may or may not be attached to a city) that forms the inner defenses, to which an army could retreat to if the city walls fell.

However, a citadel can also be used to describe a part of a castle: another layer of walls between the outer walls and the inner keep. This is, however, a more rare use of the term.