Im gonna assume you are talking about a "standard" roman Castra, Like the one from wikipedia a littel further up this thread. I was actually building something like that and planning to put it up on the forums, as a suggestion for dwarven military camps in forests and flatlands. but, as allways with my projects, i got stuck, bored, or just had other stuff to do...
I did get a few things done though, that i can share here:
1) step 1 in building a castra is to channel out a rectangular area, the size of your fort. This is mainly for removing murky pools, but it also makes it a little more dwarven. I still have to find the right size and ratio, but im hopefull 75x55 can accomodate 200 dwarves. (height by width)
2) Build fortifications on the edges of the fort, leaving only 1 enterance, and construct a walkway on the inside for your Marksdwarves. Notice how the fort from the screenshot is flawed: the walkway is to close to other constructions, and therefor military station orders would not always end up next to the fortification.
3) Build barracks. There are pictures of them further down. Make sure to leave 4 spaces between them and the wall, so that there roofs dont conflict with station orders on the walkway. The barracks are built in the upper left and upper right corners of the fort, with 3x4 barracks in each corner (height by width). this should leave room for a 5 tile wide road in the middle.
4)build catacombes. some sort of structure at the north end of the fort, which serves as enterance to below ground tombs and graves. if the aquifer is low enough, these tombs can extend below the fortifications and outside the original area.
5) set up traning areas and restrains in the lower right and left corner. Restrains can be used for either justice or live training.
6) Build command structures in the middle. Its not clearly defined what you want here, but you can start with a 13x13 building around the middle of the fort, and can then go up, down, left or right for another structure that size, or several. I recommend having at least a hospital in one of them, but dining halls, commanders quarters, workshop areas and more are available. They just need to fit into a 13x13 area.
7) your basically done now. tavern recruitment offices for auxilia troops, towers, Ballista emplacements and much more can be constructed, but thats up to you.
Screenshot with wrong size/ratio:
(https://i.imgur.com/GH05va7.png)
(https://i.imgur.com/95gpQo9.png)
And how the barracks are supposed to look:
(https://i.imgur.com/HbON8Eh.png)
(https://i.imgur.com/VgNOeuv.png)
Barracks: Barracks are made by first channeling out a 6 tile long channel. on the z level below, the upper 5 tiles are mined out. next, we construct a 5 tile long wall on top of the channel, and ramps on each side of the wall. Thats basically it! there is now room for 5 beds on each side, meaning it can house an entire squad. Of course, som barracks may serve as temples or storage instead. If you want, you can improve uppon the barracks by replacing the dirt walls with wood, and adding a wooden floor. maybe even a trap door on the entrance. (Thx to Baffler for suggesting this design!)
Thats what i have for now, I decided it was better to post it here, since i probably wont finish it, as always ;) I hope it answeres your question.