Pueblo Bonito is the masterpiece — a D-shaped great house of around 650 rooms rising four stories at the back wall, built and rebuilt between roughly AD 850 and 1150. From the trail you can walk through the original doorways and stand inside ground-floor rooms still roofed with the original pine beams, hauled from mountains more than 70 miles away.
The walls are a study in Chacoan masonry: each course laid in carefully sized sandstone over a rubble core, the stonework getting finer and more decorative through successive building phases. The two great kivas in the central plaza are large enough to walk into. This is the building that defines Chaco; if you do nothing else, spend two hours here.