Medieval English kings were usually itinerant in their kingship, travelling round their realm. King John changed location 2,702 times during his reign, staying less than three days in any one place in 88% of cases.^[1] However, not all of the government travelled with the king at all times; by 1200 the Exchequer had a more permanent base in Westminster to undertake both it's duties in collecting revenues for the king, and acting as a court (The Exchequer of Pleas).^[2] With its revenue collection duties, this would have been one area that the King's money was stored away from his person.

However the major site of the English treasury was at Winchester.^[3] It was here that the majority of the King's money was stored, and in the first part of the 12th century it was a centre for governance more generally. The importance of the site can be seen in the frequency with which John visited it; 3% of his reign was spent there (176 days), the second greatest amount of time after Central London's 7% (378 days).^[1] It was one of the first places that Stephen visited in England prior to his coronation, where he was given control of both the castle and the treasure of Henry I.^[3]

It was ideally situated - within the heartlands of the English crown, attack or raid from a foreign power was unlikely. Additionally, it was close to Portsmouth, the usual site of crossing to the Crown's continental holdings in Normandy - allowing the king to collect the required finances for war on his way to Portsmouth. The relative proximity of London also allowed transfer of revenues collected by the Exchequer there.

Treasure that travelled with the king was vulnerable - shown most capably by the loss of the crown jewels, along with other treasure, to the rising tide at an East Anglian estuary known as the Wash under King John in 1216. However, even the castle at Winchester offered limited security; during the civil war between Mathilda and Stephen the castle was sacked, both the treasure and the fort changing hands multiple times over the course of the war.^[3]

It is important to note the differences in the way the crown raised money from governments today. Now, governments collects taxes, both throughout the year and at specific intervals, to be spent throughout the year, as part of an overall budget. In Medieval England, large taxes (the geld, carucage, taxes on moveables etc.) were collected irregularly for specific purposes - generally to pay for a war, although raising money for the ransom of King Richard was a notable exception - rather than as a matter of routine. As such, the largest concentrations of money were not left around to be stolen - they were raised, possibly stored for a short period, but then spent on whatever the expenditure was. The castellan of your treasury castle going rouge, or bandits, or even a foreign force, were less of an issue than it might first appear - the money was not left there for it to be stolen in the first place.

1. Kanter, Julie, Peripatetic and sedentary kingship : the itineraries of the thirteenth-century English kings, (Kings College London, Unpublished doctoral thesis; 2011), 119-22
2. Kemp, Brian, "Exchequer and Bench in the Later Twelfth Century – Separate or Identical Tribunals?". The English Historical Review, 88 (Oxford University Press, 1973).
3. Dalton, Paul & Luscombe, David; Rulership and Rebellion in the Anglo-Norman World, c.1066–c.1216, (Ashgate, 2015).

As a followup I am wondering if these were definite targets of attack for invading forces, or if they were too difficult logistically to deal with and were just a secondary or tertiary item of interest.

Sometimes metal detector enthusiasts find coin hoards in fields. Though not knowing the owner we can't say if it was royalty but there was a practice to bury and hide wealth (and in these finds either they died or lost where it was buried)

One medieval hoard was estimated it could have been buried when there was a conflict amongst the Lords in the area.

http://www.caerleon.net/archive/miscellany/hoard/index.html Local metal detector enthusiast, Sean Flook, discovered a hoard of over thirty medieval silver coins in the Caerleon area. It is interesting to speculate how and why the coins ended up where they were found. They mostly date from the 14th century, the most recent being 1399 at the latest. Now, the early 1400s were a turbulent time in Wales. Owain Glyndwr (supposedly a descendant of a long line of British kings) led a successful rebellion against the English/Norman Lords. The insurrection started in North Wales, and in the summer of 1402 Glyndwr's army burst into Gwent. According to "Adam of Usk" Owain captured the castles at Usk, Caerleon and Newport and burned the towns to the ground. Owen's arrival in Caerleon must have been nervously anticipated, and could easily explain why the coins were buried. What became of their owner we can only guess.

In 12th Century England, revenues did not necessarily flow back to a central royal treasury. The sheriffs could be sent writs requiring them to spend revenues locally. They'd bring documents to the semiannual exchequer audit to justify the deductions on their farm. Examples that Warren gave were along the lines of "For 2 cartloads of lead to repair the roof at the manor of Foo, N pounds. For 50 hams sent to the garrison of Bar, N shillings."

At the Popes Palace during the Avignon Papacy in the 14th century, the lower treasury had secret compartments built into the floor where valuables were stashed. Reference