Kuélap
Archeological site
This was the largest city of the Chachapoyas culture and housed up to 5,000 people at its height. Built between 900-1100, it was a thriving pre-Inca culture that lived high in the mountains. Some parts were built as early as 600 and other parts were still under construction up to the Inca domination in the 1470s. There are more than 500 buildings that have been restored and a full visit takes at least 4 hours.
The remoteness has kept it relatively under the radar, though locals are working to promote it as the next Machu Picchu. The cable car (details below) has gone a long way in making it more accessible. Trails are well marked and you can visit without a guide, though a knowledgable guide makes the visit much better.
This is a very thorough article in English about Kuélap.
Practicalities
If you really want to see all of the ruins, plan a full day to visit Kuélap, including transportation from Chachapoyas. If you’re visiting from elsewhere, don’t plan to travel to Chachapoyas and also have time to visit Kuélap on the same day.