Santa María is the least spectacular of the ruins on the joint admission ticket which, combined with its out-of-the-way location make it a rarely visited attraction. A sizable plaza is the main feature, with the church very ruinous. The settlement was large, with printing press and prison; the chapel is a 20th-century addition. It’s a relaxing place surrounded by jungle that’s great for birdwatching, with toucans and trogons easily spotted.
The ruins are on the RP2 between Concepción de la Sierra and San Javier, 110km southeast of Posadas. To get there, take a bus from Posadas to Concepción. There, change to a San Javier–bound service and ask the driver to let you off at the ruins, 25km down the road. You can get a San Javier–bound bus direct from Posadas, but not all run via the ruins: check.