If you have ancestors that served in the Spanish military, this is the place to go.
Be warned, even though the archive is open from 9:00-2:00, unless you make an appointment you cannot enter until 11:00. You will need to bring your passport, and cannot take pictures. They provide photocopies (for a small fee) and occasionally make digital copies.
The archive is located inside the castle at Segovia, home of Ferdinand and Isabella. It cost money to enter the castle, but if you tell then you’re going to the archive to do research, they’ll let you in for free.
I suggest paying a few euros to get a ticket, as the inside of the castle is incredible. It was undergoing renovations during our visit, but there were signs pointing the way up to the archive.
Also, tours of the archive can be purchased for groups of five or more. This is a great way to see how a national archive functions, and gain a thorough understanding of what this military archive offers.
Once inside, you’ll be given a researcher tag, and shown to the reading room. Bring your passports along, as you’ll need them again to fill out a form allowing you to request records. The archivist was extremely helpful. We knew that a certain family had all served in the military, and by giving him their names and birthdates, he was able to locate them in the archive’s index and call up their military files, which contained enrollment information, progression through ranks, any correspondence from or about the ancestor, promotions, and discharge information.
We were able to read through them at our leisure, requesting additional files as new evidence emerged, and marking with (provided) slips of paper the documents we wished to be photocopied.
The time between when we brought our marked packets to the desk to the time the photocopies were in hand was roughly fifteen minutes, but they told us that the wait was unusual, all of the staff was busy on other projects.
As a reward for your research efforts, I highly suggest driving down around the castle to the park underneath. The view is stunning.