La Rosada was the original name of the first steel foundry in South America. It may be primitive compared to modern technology, but it was quite modern at the time.
Much of it was destroyed during the Triple Alliance War, but it was functional from 1850~1868.
The local mountains were mined for the raw ingredients. Many of the structures are still standing even though they are not complete. For example, the water canal and waterwheel are still there. The mineral ramp and loading chute are mostly still intact.
The park closed at 4:30pm when I visited (winter). However I arrived at 4:45 and the guard still let me pass. I walked around for about an hour and simply waved good bye as I left. They are very laid back so I'm not even sure if there is a ticket cost.
The national park of Ybycui is well known relatively speaking for parks in Paraguay. The infrastructure may not be the same as many international visitors may be expecting.
However the trails and parking area are relatively well maintained. I did a couple of the short hikes, but there are many to choose from and waterfalls too. Unfortunately my camera battery died, so I cannot share any photos of them.
There was a guard house at the entrance. I don't know if there is a cost or not, but the guard just waved me through since it was late in the afternoon.