According to most scenarios, he wouldn't bother
(Edited commentary): A nature of the D&D game is that illusion and detection spells are in an arms race to reduce the prevalance of scry & die tactics. From a tactical point of view, the resources spent hiding a fortification are either better spent hiding something smaller but more secret or fortifying the same structure against expected enemy tactics. The lowest item cost is approximately a hundred thousand gold, and that's for a custom item that the DM may not allow. The only time when concealing a fortification makes sense is during its magical (don't bother if you actually have to hire people to construct it) construction in a barren wasteland/desert as part of the first stage of defences against an enemy able and willing to use armies. Rendering a castle hard-to-detect takes more than visible illusions.
The most effective anti-detection methodology is to not be there in the first place. Enchanting the ability to planeshift/teleport/fly into the structure is a necessary pre-requisite to hiding the structure, as without that enemies can rely on fairly simplistic means of kinetic detection. or the spell "Find the Path." As most structure-hiding spells are at 5th-6th level, find the path must be factored in as a potential threat.
Once the structure is enchanted to move (random-walk if possible, which is why the best method is some sort of planar travel), the first layer of detection-defense must be enchanting false vision into every part of the structure. Again, without considering metamagic cheese, a tower of london structure has a volume of approximately 1136340 cubic feet. (Not counting outer fortifications). Every casting of false vision consumes 268083 cubic feet, so 5 castings will be necessary to cover the entire structure. This will protect against trivial scrying, but not find the path.
The probable line of attack, given that the structure is on a random plane, is to use contact other plane or its equivalent to have an outsider answer "Is $location on $plane?" Repeat for a number of castings. Said outsiders may or may not know the answer, but given that the caster is contacting notable powers on the plane and that the structure may impinge on their domain, there's no reason why they wouldn't know the answer. Still, this forces the enemy to use non-trivial resources. However, neither screen nor false vision will prevent find the path from working. Happily, find the path will not provide a useful teleport target and false vision is sufficient to block simple scrying sensors sent upon the path. (As a note, obsucre object could block find the path... if it could be scaled up to cover a location. It would be worth doing original research so that obscure object is a cubic spell, rather than an object spell. Barring that, as that's a house rule, detect scrying might detect the find the path sensor, but probably wouldn't. Nondetection functions upon "object touched" and certainly prevents divination... but not without a chance of failure. It's worth enchanting it into the castle at an extremely high level. Private sanctum fails due to Find the path not being a scrying subschool.
Fundamentally, the only real protection against find the path is a fairly frequent random planar walk of the castle combined with a high caster-level nondetetion and implemented "false vision" spells.
That being completed, the logic below is valid.
(End edit)
There are a number of options here, depending on your budget.
The simplest method is to simply use Mirage Arcana enchanted into an item. At base caster level (not thinking about cheesing items here, there are rather large resources devoted to that act) a level 5 spell requires a level 11 casterlevel.
This provides for a surface area of 20 feet * 11, or 220 feet that needs to be renewed every 11 hours.
Looking at a fairly trivial example of the tower of london, a nice square keep.
Dimensions are here: roughly 110 feet per side
The White Tower is somewhat irregular in plan, for though it looks so square from the river its four sides are all of different lengths, and three of its corners are not right angles. The side towards which we approach is 107 feet from north to south. The south side measures 118 feet. It has four turrets at the corners, three of them square, the fourth, that on the north-east, being circular. From floor to battlements it is 90 feet in height. The original entrance was probably on the south side, and high above the ground, being reached as usual in Norman castles by an external stair which could be easily removed in time of danger. Another or the same entrance led from an upper storey of the palace. The interior is of the plainest and sternest character. Every consideration is postponed to that of obtaining the greatest strength and security. The outer walls vary in thickness from 15 feet in the lower to 11 in the upper storey. The whole building is crossed by one wall, which rises from base to summit and divides it into a large western and a smaller eastern portion. The eastern part is further subdivided by a wall which cuts off St. John's Chapel, its crypt, and its subcrypt, each roof of which is massively vaulted. There is no vaulting but a wooden floor between the storeys of the other part. There are several comparatively modern entrances.
So 110x90 for surface area, or 9900 square feet per side. A casting provides a surface area of 220 square feet. Therefore, with this el-cheapo "put the spell inna stick" method, costing (as per here) command word activated, 11*5*1800, or 99000 gold, It will take a minion roughly 5 minutes per side to fully renew the enchantment. It's reasonable to have 4 watches and to have the captain of the watch renew the enchantment every watch.
Another option is the use of a bunch of scrolls of Permanent Image. Best incorporated into surrounding terrain, you can simply disguise your castle as a hill. While you'll want skills in some sort of artistic craft, creating a bunch of permed images of "portions of a hill" should function adequately to your purpose. The cheapest option (besides from spending a non-trivial amount of time casting it yourself) is with a spell-trigger item. At a 6th level spell, it can create image covering a surface area of 150 feet. To cover the 5 sides of the tower with believable "hill" coverings, you need to cast it 330 times, which means it's probably just more effective to put it in a command word activated stick. This has the benefit of not needing regular re-casting.
However, we're just getting started.
With a little more imagination, it should be possible to build the castle out of invisible stone. One gets invisible stone through judicious use of the "wall of stone" spell and the Invisible spell metamagic:
Benefit: A hidden spell incorporates an illusion that renders it and its effects invisible. Invisible Spell does not conceal the casting of the spell, only the spell effects. An invisible spell uses up a spell slot three levels higher than the spell's actual level.
Notes: If the spell reasonably makes noise, has odor, or a tactile effect of some kind these effects are not hidden, it operates like any other invisibility effect. The “spell effects” refer to any magical manifestation of the spell, not indirect results of the spell. For instance, a ball of fire would be invisible but anything it ignites would be visibly burning.
As a sor-wis 5, spell, an invisible wall of stone is 8th level unless one employs typical tricks for reducing caster level of meta-magic. Still, this means that the fundamental infrastructure of the castle is invisible, reducing the problem to rendering the inhabitants invisible. A far more solvable problem.
Assuming one enchants a stick of permanency, a sor/wis 5 spell, at casterlevel 10 (it's more effective to do casterlevel 13 so you can make more things perminant) you can simply enchant a stick of invisiblity and render every stone and every item coming to the castle invisible, and grant all inhabitants a free see invisiblity.
A stick of permanancy costs... 5 gold per 1 XP on top of the base price, or roughly 340,000 gold for a stick at caster level 10. A worthwhile investment, however. The way to pay off the investement is to bring the minimum caster level to 12, so you can create endless walls of fire, for 608,000 gold and thereby dump functionally unlimited energy into the local civilization, with a concomitant increase in living standards.
There is, of course, the thoughtbottle option, but some things are too cheesy even to suggest.
All things being equal, the best way to hide a castle is to build it inside of the landscape and hide the bits that you changed with perm image. Beyond that, an item of mirage arcana and 4 shifts of guards should serve to hide the castle. The problem with this is that the castle is still vulnerable to scry&die and the most trivial examination of the local terrain (due to the natural layout of castles and their infrastructure (read: villages) ). While the infrastructure problem is also solvable, at a certain point it may make more sense for the castle to be flying (arms and equipment guide, probably, to which I don't have access right now, or the stronghold builder's guidebook).
Both of those books probably also detail additional building camouflage techniques, though of dubious utility.
Remember, when trying to hide a castle, you need to make sure it can hide against the expected threat. Make sure to perform a threat estimate before committing any gold, because most of these methodologies can be defeated by fairly trivial divination that a party of sufficient level to assault the castle will have access to.
Good luck :)
