| MDSS |
There is a whole Downtime ruleset for this sort of thing.
you can't "craft" a castle. Guédelon, in France, started construction (using only medieval techniques and tools) in 1998 with a 50men team and it is STILL under construction by 2023, using a full construction team, so "crafting" it with a single character is ludicrous, even if above ground.
You can, however, use spells such as Stone Shape, Rock to Mud+Shape Earth+Mud to Rock or Create Holds to pretty much create and shape a cave however you like.
Finally, there is the Lyre of Building.
"Once a week, its strings can be strummed so as to produce chords that magically construct buildings, mines, tunnels, ditches, etc. The effect produced in 30 minutes of playing is equal to the work of 100 humans laboring for 3 days. Each hour after the first, a character playing the lyre must make a DC 18 Perform (string instruments) check. If she fails, she must stop and cannot play the lyre again for this purpose until a week has passed."
Note that the DC does not increase, nor is there a time limit for how long you can play. As long as you are wearing a Ring of Sustenance as well, you could potentially perform for 21hours and a half, for the effect of 100 humans working for 69 (nice) days, that is, once a week you could take a day of downtime to build the equivalent of two and a third month of work... do that for the 52 weeks of an year and you built the equivalent effort of 9.8 years... now building a castle isn't as ludicrous anymore. in a single year you would have reached Guédelon's current state, in another half year at most and you could have built a castle... above ground. To make it underground I would recommend either looking up for how much a mine expands per year, or finding/creating a sufficiently large cave to build your castle inside (probably one where the towers in your walls go all the way up to the roof, though I wouldn't completely block the air circulation by having the wall itself touch the roof. Unless everyone inside the castle have rings of sustenance, are undeads or can, otherwise, ignore breathing.
I will point out that if you are building underground, you probably don't need to worry about buying stones, and that, while the "bard" is building the castle, any caster should be perfectly able to mine the cave itself for rocks, so the Lyre don't need to diverge it's work force to aquire the stones. Or you could get a second Lyre.
finally: the skill to build a castle should be Knowledge (Engineering), not Craft (Castle).
ps: you should take a look on the D&D 3.5 suplement for castles mentioned on a comment above if you want to actually map out the castle. It comes with rules for using the Lyre of Building, though the Perform DC of theirs increase per hour of continual use and have an in-built limit of 8hours per day, ignoring any interactions with Ring of Sustenance or the like.
Edit: f***, the button to reply is ABOVE the comment, not under it? damn, I replied to the wrong person =p