These guys here appear to be working on some really interesting technology we could tap into to get the castle resupplied from low Earth orbit.
A couple of tidbits from their web page:
Our goal is to deliver propellant to Low Earth Orbit (LEO) at less than 10% of the current cost. By using hydrogen gas gun launchers we will attain costs of $500/lb at the pump in LEO
Our solution blends a Jules Verne/Robert Heinlein concept of gun launch to space with 21st century hydrogen gas gun technology.
I read about that system a few months ago. It's an interesting idea. Basically, you build a big gun. In this case, the extra long barrel is supported by mid-ocean deep waters.
Pump the barrel full of hydrogen and air, or hydrogen and oxygen, or propane and air, (and so forth), put in a launch capsule, and light it off.
The acceleration is tremendous, and the g forces on the capsule are quite extreme. That's why the proponents suggest using it to send fuel and other substances that would not be crushed in the acceleration.
It reminds me of my suggestion about using an extremely long launch rail and magnetic coils to launch at a more moderate acceleration.
My proposal was to use Antarctica as the launch point, and to use ice as an ablative material, because this type of launch hits maximum resistance as soon as it clears the muzzle. Many people have suggested that my scheme would be impractical, but I simply respond that you have to think big.
If you scale it big enough, it has to work.
One aspect of my suggestion may well be impractical, though not necessarily impossible. I also proposed to send large chunks of relatively pure ice into suborbital trajectories, and disintegrate them with onboard demolition charges at the appropriate re-entry altitude. My purpose for this was to be able to deliver rainfall at any location on Earth, on time and on schedule. We could make deserts into food-producing regions without any land-based infrastructure.
It appears that my rather ambitious ideas may have to wait for a new generation of ambitious people unafraid to dare large dreams.