[Tyhm]

Expansions to the idea:
First, I chose 100 gralats for doing it because it's going to be really difficult and because if it's not a really good prize for doing it/a really bad penalty for blowing it, kingdoms would put their own men on the wagon and crash it themselves. Or just have the "guards" do a horrendously crappy job, since I assume the guards wouldn't be able to damage their own charge, simple flag detection, anyway.
The bank itself would have a counter for players without kingdom affiliation to deposit and withdraw, and (of course!) rob. Naturally if you rob the bank every player in the game is after you for your money, and so are all the police (I believe they are set up so you can add code like this to all of them at once...)
Also, each castle would have a local bank, where a little old man takes your money and keeps track of your account, depositing and withdrawing from a tiny safe in the back. The wagons ALSO carry the money too and from the kingdoms! The local bank only holds maybe a thousand gralats, when it goes under they send a request in the morning that the bank send them more from the Kingdom's account, when it goes over they send the surplus with the evening shipment.
So:
if the King of Dustari wins a war against the Samurai and wins 500 Gralat, he deposits it in the bank. The local bank was down to 850, + 500 they're at 1350. At 6:00, the wagon from the Graal bank shows up, and the little old man gives them the other 350 gralats to put in their account. The wagon men ride back with an escort of Dustari guards. The pirates know about this and attack in mass, slaughtering the Dustari guards and making off with the 350 gralats. The Dustari's bank account at the Graal bank suffers.
The next evening at 6:00 the Pirates' little old man gives the wagon man the Pirate surplus, 500 Gralats (150 from Ghost Pirate's deposit + the 350 from the raid). The Dustari ambush and steal the Pirates' treasure, and try to get it back to the bank the next day!
If the Pirates successfully deposit their gold, the Dustari are in bad shape. If they all try to withdraw their money, they can't! They can only get 1000 from the Dustari bank a day, and they can't get much back from the Graal Bank in the morning shipment because they never successfully deposited anything!

This would give the players a bank to store their money for when they die, it would give the kingdoms a basis for a unique economic system, and most importantly it would give the kingdoms a really good reason to beat the crap out of each other on a daily basis!