These extensive Inca ruins perch on a swathe of barren pampa (pampas grass) at 3700m. It’s a worthwhile two-hour trek on a steep path from behind the central market in La Unión. There are an incredible 2 sq km of ruins and supposedly more than 1000 buildings and storehouses in total remaining here today. The most impressive structure is the usnu, a huge 4m-high ceremonial platform with engravings of animals (monkeys with lion faces) adorning the entrance.
Huánuco Viejo was defended by Illa Tupac, a key figure in the Inca resistance against the Spanish, until 1543, significantly after many Inca settlements had fallen.
Minivans leaving from the market throughout the day (S3) can take you to within a 20-minute walk of the site.
It's a 1½ to two-hour walk from La Unión, starting from behind the central market, where a steep flight of steps lead up to a water tower. From there, the defined path continues through beautiful altiplano (Andean plateau) landscapes. Once you've climbed up to the plain, look for a road and the ruins visible on open ground to the right. Locals will point you in the right direction.