The park is named after a hilltop fortress built in the 1762 by the Portuguese, early captured and finished by the Spaniards. The place has witnessed battles between Spanish and Portuguese, Spanish and British, and civil wars. The fort has been restored and is one of the attractions to visit in the park.
Santa Teresa has wide area available for tent camping, and has some bungalows to rent. It's very popular among camping fans. It's ridiculously cheap to camp there, one parcel that can be used by up to 7 guys, costs 150 pesos, about 6 american bucks. Being so affordable it gets crammed with youth in summer, well, not really crammed, the place is 1054 hectares wide (2064 acres), so it's big enough to host an army of camping fans. The beaches are great, oceanic, big waves (in uruguayan terms) I'd say waves up to 2 meters tall, surfing is a popular sport there.
The place is run by the army, and management is not very good. The supermarkets (there are 2 of them) run out of supplies rather quick, and if you ask "where can I find a free spot to camp?" you won't get very clear directions. On the other hand the place is safe like no other, and the park is very well taken care of.
Santa Teresa is a very popular destination for the uruguayan middle class, surfers, and teenagers, providing a good blend of natural resources, peace, safety, adventure and low costs. It's a humble type of tourism, the spirit among the campers is of camaradery, usually older campers (families) lend the tools to the groups of unequipped teenagers that try the camping experience. It also attracts a great deal of tourists from the south of Brazil, and from Argentina, in particular people from Cordoba.
The park is only 5 km away from Punta del Diablo, another seaside tourism hotspot in Rocha, and about 30 km to the border with Brazil, El Chuy, a border town with half a dozen of duty-free shops. Actually the trip to Punta del Diablo can be done by the beach which is one of the popular things to do there, in a walk of a couple of hours.
It's a place I'm very fond of, which I visited all my life, first as a kid with my parents, and later on, in my teenage years, we would go there with a bunch of friends every summer. When I think of Santa Teresa, lot of memories come to my mind, memories of campfires, beach soccer, walking in the beach for hours, fetching wood, making up excuses to visit the girl's neighbor camp, the night as one mantle of stars above the trees, hideous toilets, hitchhiking back to Montevideo for hours, and so on.
Here is a photo album that I hope will help you understand what's Santa Teresa like,