Honestly, I would just copy down the worldgen information, regen your world, and start over from scratch.
A flat desert is probably not the best place to put a castle, honestly... you want the high ground, after all, it prevents the seige weapons from being easily carted up to your walls.
Still, if it's near a river, you can divert part of the river in a large loop, and make a "moat". If that's all sand, then all you have to do is dig a big channel with a couple ramps leading upwards on the outer end (just in case a dwarf falls in), and it shouldn't take your miners very long at all to complete.
Unless you want to later go in and dig a tunnel to some extra lumber areas (or just dig another moat around the area and make a drawbridge), you would want to make this area very large, so you can get some in-the-fort lumber production going, although that depends on how much vegetation you can even get, and whether or not to just rely on importing it from the trade partners. Obviously, if you have terrain features you want to keep, like a magma vent, you want that inside your castle, as well.
If you want to be even more authentic, keep in mind that it's pretty much not possible for pure stone castles to be more than 5 stories tall, and every stone wall will need to have a wall below it. Technically, you might want to make the walls even wider as they go down. Floors that you construct should be made of wood, you just can't support stone floors without multiple supports or arches. Wooden walls should be fine, so long as they aren't too large or far away from a supporting wall.
One of the beauties of castles, though, was that they allowed for very small garrisons to defend against vastly superior numbers. Many castles were designed to be defended with only 30 soldiers against as much as a thousand attackers. (Which is pretty much why seiges are a joke in DF - for traps to be balanced out, you would need to have hundreds of attackers storming your gates.) They were just too expensive to build to put too many people inside. So just modify your init file, and make sure that the population limit is really small... you want a baron, because what's a castle without a noble, but beyond that it's too much, so set the limit to something like 80 or 100 dwarves.
... Hmm... I'm really getting into this, I have to make an above-ground castle, now.
I'm going to go genning looking for a place where I can build a castle on a mountain overlooking a human trade road, as part of a strategic defense and/or threat upon a powerful human empire.