The question likely says it all, but, in the treasure room in the Sanctuary is a key up high. As you gather money, the pile grows and you can climb on it. I'm wondering exactly how much gold you are required to accumulate to be able to reach the key. I assume the key opens the chest in the same room at the bottom of the pile.
I believe the number is 5 Million. But here is the thing. Once you have that much the chest will be covered in gold and inaccessible.
To get rid of it all you have to give your money to the royal treasury which you can do if you are the king. If you aren't there yet keep playing.
3Finally got to this point. Noticed two things. 1. The amount seems to be 5 million. 2. You want to do this before the big event that 'Ends' the game. After that point, that key is no longer present for you as I found out unfortunately. I didn't earn the money until after the end game. Nov 3, 2010 at 18:11
1@Blake you are correct, after exhaustive testing (read: 5,000,000 and 4,999,999) I have determined it to be exactly 5 million.– tzenesNov 5, 2010 at 4:55
@Blake the key appears to be gone but you will have access to the chest when you have got rid of all of the money in your personal stash.– kymullyNov 24, 2010 at 3:34
The number is exactly 5 million. The key will appear to disappear when you reach this amount, but it is not treated as a regular gold key you can pick up and store. When the key has disappeared, this means you can successfully open the chest if you get rid of all of your money.
You don't have to get rid of all of it to access the chest, but I think anything over 100,000 in your personal stash renders it unobtainable.