[Tuidjy]

When you teleport into a province that contains a castle of yours, you
end up in the castle. If a besieging enemy does not assault the fort,
there will be no battle. If the province is yours, an enemy invades
it, and your troops teleport on that turn, they will not participate
in the battle for the province.

That is just the way it works. If you have to choose between the two
alternatives, this is the better one... as long as you know how it
works, and take advantage of it.