So it's a raid after all. The Mekia Holy Land theocracy really did send a disguised force all the way to the other side of the Empire, to launch a surprise raid on a fortress defended by elite knights. That's crazy, but I guess the point may be to demonstrate that Mekia and its leaders ARE crazy. And the ones sent to raid it are strong enough to do it that way, without the help of things like siege weapons.
What would be the strategic objective though? Pin the blame on the Kingdom? Or maybe weaken the Empire to prepare for another attack?
I guess the point of Amelia's attempt to seduce the knights was to confirm they've found the targets, and gain easy entrance to the fortress if possible. She's the leader of the Mekia raiding force, so that was a risky move for her to play that role.