Stronghold Builder's Guidebook has gotcha covered. Except, well, it's now an out-of-print official 3e D&D supplement, from Wizards of the Coast back in 2002. You might be able to find a used copy somewhere online like Amazon or E-Bay or whatnot, I dunno, or in a used bookstore perhaps.
I could do the calculations and whatnot for you, but it'd take awhile and I'd need very specific details on the size and number/type/quality of rooms in the castle and how many guards, how many nobles, and how many other people it contains (plus how many and what quality of guest-rooms, if any), etc. Materials, magical properties, etc.
Mobile strongholds come in 12 types: Astral (move stronghold back and forth between the Astral Plane, stationary unless you pay for a movement speed), Burrowing (stronghold moves through the ground like an earth elemental, requires paying for a movement speed), Crawling (stronghold crawls or slides along the ground, requires paying for a movement speed), Ethereal (move stronghold back and forth between the Ethereal Plane, stationary unless you pay for a movement speed), Flying (stronghold floats in midair at a fixed height and position unless paying for a movement speed), Inner Plane-Linked (move stronghold back and forth between a particular place in a particular Inner Plane, protects against the plane's environment, stationary unless you pay for a movement speed), Outer Plane-Linked (move stronghold back and forth between a particular place in a particular Outer Plane, stationary unless you pay for a movement speed), Plane-Shiftnig (allows the entire stronghold to Plane Shift as per the spell to other planes, stationary unless you pay for a movement speed), Sailing (stronghold floats on water but can be moved by currents, otherwise stationary unless you pay for a movement speed), Shadow-Shifting (move stronghold back and forth between Plane of Shadow, stationary, must pay for a particular number of hours per day that it can remain in the Plane of Shadow), Submersing (stronghold withstands pressures of up to 20 miles underwater, stationary on the seafloor unless paying for a movement speed to let it move like a submarine), or Teleporting (stronghold may Teleport Without Error (or Greater Teleport in 3.5 rules) one or more times per day based on how much you pay). The Shifting mobility effects are limited to 5 shifts per day and require paying an appropriate amount for each shift the stronghold is able to perform per day.
Of course, as the DM you don't need to worry about the cost of the stronghold. The costs and prerequisites in the book are there for PCs who want to make their own strongholds. But each form of stronghold mobility does require enchanting the entire stronghold like a magic item, using the Craft Wondrous Item feat and certain spells and paying a certain cost, with the cost based on just how big the stronghold is. Stronghold locomotion is measured in the book by miles per hour or a fraction thereof, which determines the price of any given locomotion for the stronghold (based on size, again). Mobility forms like Crawling or Sailing are relatively cheap (1,000 or 3,000 GP per "stronghold space," respectively; stronghold spaces are roughly 20x20 feet with a 10-foot ceiling) and can be infused in a stronghold by a low-level mage or priest (Sailing requires the Water Walk spell, while Crawling requires Augment Object, a spell from the SBG itself, available to most primary spellcasters in the core rules).
