This deserted fortress is a fascinating place to visit. BLM maintains an easy 1.5 mile trail to the ruins with lots of signage. The roads are not marked so you have to know what you're seeking. Reminds me of petroglyph sites and it might be to help preserve the site.
If you would like directions, feel free to email me.
While it was built by the same Irishman who started Tucson's presidio, this fortress was built in 1776 and was abandoned after 5 years due greatly to attacks of the Apaches.
Here's the BLM description of the site:

"The Presidio Santa Cruz de Terrenate is the most intact example remaining of an extensive network of similar fortresses that marked the northern hegemony of Spain in what is now the State of Arizona. The Presidio was established in 1776 by an Irish mercenary, Hugh O'Conor, for King Carlos III of Spain. It was never completed and was abandoned after 5 years.
'The terror instilled in the troops and settlers at the presidio of Santa Cruz that had seen two captains and more than 80 men perish at the hands of the enemies in the open rolling grounds at a short distance from the post, and the incessant attacks which they suffered from numerous bands of Apache who do not permit cultivation of the crops, who surprise the mule trains carrying effects and supplies, who rob the horse herds and put the troops in the situation of not being able to attend their own defense, making them useless for the defense of the province.'"