Remote Forte Príncipe da Beira, beside the Rio Guaporé, 210km south of Guajará-Mirim, was constructed by the Portuguese between 1776 and 1783 to consolidate their hold on the lands east of the Guaporé and Mamoré against the Spanish. The star-shaped fort, one of only two ever constructed by Portugal in the Brazilian interior, has 10m-high walls and four corner bastions, each of which held 14 cannons. Today just one cannon remains.
The walls, nearly 1km around, are surrounded by a moat and enclose the ruins of a chapel, armory, officers quarters and prison cells in which bored convicts scrawled poetic graffiti. Underground passageways lead from the fortress to the river. The fort was abandoned as a military post in 1889. Today Brazil maintains a garrison of around 70 soldiers beside the fort. There’s also a village, Vila Príncipe da Beira, here. The town of Costa Marques (population 7000), about 25km from the fort, is home to an attractive church, orchid park and a turtle nursery. The road to Costa Marques is impassable from about November to April.