Step One: Land 40K CS in donut townsland as a single unit, unannounced and unheralded.
Step Two: 
(March on the capital, which is cut off from its armies)
Step Three: Profit
1. Aurvandil's entire army is only 25kcs and that's counting militia. Only Astrum has 40kcs, and chances are a good portion of that is militia.
2. The "Doughnut" region is almost always a fortified towns-land with militia and units moving in and out of the capital. The landing force would suffer heavy casualties.
3. The capital would likely have heavy fortifications, militia and more then a few units. More heavy casualties.
4. As mentioned, this would leave Aurvandil completely open to attack.
5. If under different circumstances a realm did launch a massive amphibious assault on a realm's capital in a risky gambit to win a war it would be AWESOME.
Yes, SA's cohesion is just incredible. I forgot that it didn't take almost two weeks to organize a halfway decent fighting force to retake Paisly. which was essentially unguarded. I bet the 'Moot's even worse better.
Two weeks is about as quick as this sort of thing could possibly be organized. It's not our fault Terran called us in when we weren't really needed.
No matter what it's gonna be a knock-down drag-out where victory for the defenders is not assured. Through miraculous reactions from all the realm's nobles and favorable RNG they MIGHT be able to fight Hypothetical Invasion Force off, but the odds are against it.
Only if your hypothetical fighting force is more powerful then anything that actually exists, lands all troops roughly at the same time, and ignores the fact that claiming beachheads is an additional battle penalty on top of the disadvantage of storming fortifications.
Even if they manage to hold the capital the realm is far from doomed. They can simply move the capital, raise an army and take the city back (assuming the army hasn't already left by then due to lack of gold).
I have seldom seen townslands with more than 5K militia, and their fortifications won't mean a lot once the siege engines start rolling in. I don't see how a typical townsland would be anything more than a speedbump unless they know exactly what's coming, which is essentially my whole point.
I believe "navies" lets you know if an enemy force is coming, plus there are always informants and infiltrators.
Anyway, massive morale drops are a mechanic that prevents this kind of thing. Coupled with sea travel it's more important than ever.
There are already two other mechanics that prevent this from working out like your describing. First is unit payment, the invading force wouldn't be able to stay long especially with cost of a mercenary force. Second is that they could never TO the region and expect to hold it because of the distance from capital penalty. They could CTO but they'd most likely succumb to siege in a matter of weeks. If these proved to be insufficient to make such assaults acceptably challenging then the answer is simply to turn up the difficulty for landing troops on a defended coast.
Also once realms are aware of this possibility they'll start keeping larger defending forces in the capital.
I just hope everyone campaigning against the mechanic on the sole basis that SA would be able to shove more of its boot up Aurvandil's ass keep in mind that it works both ways.
If I logged in and learned Aurvandil had landed an amphibious force in Darfix or Eidulb it would make my day. See that's the problem, what you're describing is AWESOME and would be a great thing to have happen in game provided it was a suitably challenging feat. Under the current system it would still be a huge challenge to pull it off without getting massacred in the process.
