Now off it and now on; but when he saw
High up in heaven the hall that Merlin built,
Blackening against the dead-green stripes of even,
“Black nest of rats,” he groaned, “ye build too high.”Alfred, Lord Tennyson, Pelleas and Ettare
I’ll start this out by noting it’s generally understood to be a fictitious idea - or rather, the licenses were real, but their purpose was more nuanced - but it’s such an evocative and especially a gameable concept.
The core idea behind the license is that kings and other such rulers have a dilemma:
- They want their vassals to be safe. Losing the home of a vassal knight because it got captured or raided or bandits struck it is a hassle.
- But… not too safe. There’s a real danger that strong castles will eventually be used against you, and it threatens your monopoly on the means of violence.
So you issue permission - allowing you to track who is fortifying themselves, consider the merits of the case, etc. Since the difference between a building meant to be a fortification and one that just happens to be secure is kinda vague, the key identifier of these are the crenellations. Castles built without a permission are “adulterine” and theoretically subject to destruction (but by you and whose army? they have a castle!).
Actual licenses compared to castle building suggests that while royal authority and oversight over fortifications was a real concern, the crenellations or even just the act of getting the license may be more important as a symbol of royal recognition, and any actual regulation against fortifications was applied in more case by case judgments.
In The Death of Arthur
We’re playing this one mostly straight, at least early on. Strengthening your defenses above the License to Crenellate limit will be identifiable by said crenellations and may be seen as a threat if you don’t have the license.
Having a good justification, or weak central authority, may help you get a pass for your actions, or at least kick the penalty down the road long enough to get a post-facto approval for your actions (again, there’s a benefit in having a castle when it comes to not being pushed around).