If there's one thing the epic level handbook had to say, it's that price limitations are always a factor.
How about making your own demiplane and situating your fortress there? You can populate it with races you yourself create (via the creation seed.)
Adamantine is the strongest material in the game, but if invaders have their own mages, they'll be packing spells to soften it, disintegrate it, or turn it into other materials.
First two paragraphs I agree with.
The last, well, Adamantine gets outclassed by 'obdurium'(Hardness 30) from Stronghold Builder's Guide. But, just about any attacker at high levels will pack the magic necessary to neutralize the strength of walls. Even if you use walls of force.
Which is why you get an anti-magic field emanating around the exterior walls.
Lead-line
every wall and door you can afford to.
OP - What class is your character? Get Forbiddance, Mage's Private Sanctum (that's what the SRD calls it), Teleport and Dimension Door blocking magics, use permanent invisibility effects on a wall or walls of force for all the windows - for areas where you have exterior openings, ie loopholes, have them in self-contained rooms, requiring a specific key to pass through the passwall effect. Airtight the structure, have the 'back door' be a teleport or dimension door pad/trap to outside the fortress. Prismatic Walls on the exterior, with illusions over them such that they appear to be normal walls.
And remember, if someone finds the Angelwing Razor, your walls, no matter what they are, are shafted.
Basically, you
need the SBG, check out Heroes of Battle for some siege weapons.
If you want the place to be more or less impossible to take, have the primary fortress consist of a demi-plane, as suggested before, over the entire demiplane, seal it against magical insertion, except for one spot - that one spot should be where almost all the defenses on the demiplane are located; ideally you'd have the primary way onto the plane being a permanent gate or portal of some kind - treat it like a stargate, default is that you have the iris closed.
On the material plane, you want a relatively unassuming (from the outside) facility that is likewise warded against magic, and internally fortified and otherwise defended as though it contained the self-destruct button for the multiverse, or at least, as close as you can manage it. This facility should contain the other end of the portal/gate to the demiplane - and it should contain a mechanism to contact the demiplane's end, in such a way that the entry chamber is sealed off and the demiplane can see all of it.
How high a level are we talking here? That too will affect things.
