Yes I know you mistrust the banks, milord, and I don't blame you, but their Vault Wizards are specially trained to prevent dragons from detecting large amounts of gold. I cannot emphasize enough that it's a full-time job employing multiple specialists, I'm not trying to be humble here but it's not something that just the court magician and I can set up a couple wards for on the weekends and call it good.
It's, it's just that dragons are the primordial embodiment of avarice wrought into fire and flesh. They are truly, supernaturally good at finding large amounts of valuables, that's why the big mines hire those Dragon Scouts to go sniff out their lairs and mark them on the maps as potential mining ventures. You know, in case someone slays the relevant dragon. Which doesn't happen often because, milord, they are simply not that easily slain.
No I know you've hired many knights, blooded warriors and true. Yes, I was there when you gave the ten most impressive ones their special sashes. Very grand, very high honors, of course. Ehm. It's just, none of them have ever actually faced a dragon. Yes no I know Sir Edbert says he did but Sir Edbert is rather notoriously prone to exaggerated and tragically unverifiable tales---
Well no milord of course I would not doubt the word of a sworn knight. Perhaps his sobriety, but not his word, as such.
The point is that the grand treasury, while surely grand and a very special notion, is just... it is mayhaps not the ideal way of handling the realm's finances? Perhaps a series of smaller vaults, capped well below the dangerous wealth threshold at which gold is known to whet the appetite of colossal winged harbingers of death, in different corners of the realms or...?
No, I, yes well I do realize that will impede anyone's interests in coming into the vault to hurl around the gold coins and go "whee, I'm so rich!" I am aware of its deficiencies as a plan in that regard. No, I see I've misjudged a few things.
Actually, thinking on it, milord, I truly believe what you need is a fresh set of skilled wizards on this job. The court magician and I, we cannot keep up with your visionary thinking. We're too old-fashioned. But the wizards revolutionizing the eldritch academies seem to be more on this sort of level. I hear they've made some truly remarkable choices in terms of outsourcing all of their spellwork to the Ever-Whispering Void, such that it takes mere minutes for them to set up an entire defensive array. That's just the sort of innovative thinking you require.
Though it will grieve the court magician and I to leave your service, perhaps this is a sign that retirement is overdue. So I'll just... be moving further away from the big pile of gold... in the opulent, dome-shaped building with the crystal skylight... best wishes.