Las Mercedes sits at just 90 meters above sea level on the Caribbean slope, between the foothills of Turrialba Volcano and the alluvial plain. Rivers define its geography: the Dos Novillos flows 700 meters to the southeast, the Parismina 2.5 kilometers to the northeast, and smaller streams thread through the site itself. The 25-hectare complex divides into three sectors - Las Mercedes-1, Las Mercedes-2, and Las Mercedes-3 - though only the first has been extensively excavated. Within that five-hectare area, archaeologists have documented seven platforms, two plazas, six retaining walls, seven walls, four funerary areas, one ramp, and the convergence point of two stone-paved roads. The rivers that surround the site may have served a dual purpose: water supply and natural defense, forming barriers that bounded the sphere of a chiefdom.

Two stone-paved causeways extend from the heart of Las Mercedes-1 in opposite directions, each named for the modern town it points toward. The Iroquois Causeway has been traced for 660 meters before disappearing at the edge of the EARTH University reserve that now protects part of the site. The Pocora Causeway stretches more than 300 meters to the southeast, raised above the surrounding terrain as protection against flooding, before ending at a stream called Santa Emilia. At that terminus, a pair of low walls with circular halves flank the causeway, and a set of stone stairs descends to the water - perhaps once the approach to a suspension bridge connecting to another architectural complex beyond. Both roads are five to eight meters wide, paved with stone, and run perpendicular to the river drainage system. Their construction required enormous quantities of stone, fill material, and organized labor. These were not footpaths; they were ceremonial boulevards.

The central platform, now called the Hartman Platform after the Swedish archaeologist who first excavated it in 1896, stands six meters tall with a diameter of thirty meters. Carl Vilhelm Hartman, a botanist turned anthropologist, removed two 1.85-meter stone warrior figures from its summit, sculptures now housed in Sweden's Museum of Ethnography. Around the principal platform, at least fourteen others cluster in various configurations - some circular, some shaped like figure eights, one described by archaeologist Ricardo Vasquez as resembling a peanut. All were likely topped with conical huts made from organic materials long since decomposed. Hartman also excavated seventeen stone-lined graves containing ceramic vessels worn from actual use, stone celts, and a single European glass bead - proof that the site was still inhabited when the Spanish arrived.

Carbon dating and stratigraphic analysis place the earliest occupation of Las Mercedes at approximately 1500 BC, with monumental construction beginning around AD 1000. That means people lived here for twenty-five centuries before they started building the platforms and causeways visible today. The site's association with the Huetar, a Chibchan-speaking people, connects it to a broader network of chiefdoms that organized political life across pre-Columbian Costa Rica. Archaeologists who analyzed artifacts from the 2009 field school believe Las Mercedes served as the center of a chiefdom - a regional political capital comparable to the better-known Guayabo de Turrialba. European glass beads found in burial contexts confirm that the inhabitants were still present at first Spanish contact, meaning this was a living city, not a ruin, when the colonial era began.

The funerary areas tell stories that the platforms cannot. Alanson Skinner, who excavated during the winter of 1916-1917, found graves containing not just ceramics but incense burners, stone disks, meteoric iron gravers, flint knives, greenstone earspools, jade ornaments, a copper bell, carved stone statues, and objects of gold or tumbaga - a gold-copper alloy prized across pre-Columbian Central and South America. The rectangular, box-shaped graves were lined with cobblestone walls and capped with flat limestone roofs and floors. These were not hasty burials. The care taken in construction and the range of grave goods speak to a society with specialized craftspeople, long-distance trade connections, and elaborate beliefs about the afterlife. Today, part of the site is protected on the campus of EARTH University, where the jungle is slowly reclaiming what the railroad once bisected.