Machu Picchu unites on a single mountain ridge two extraordinary problem-solutions of Inca architecture: making mortarless stone earthquake-resistant, and turning a steep slope into livable, cultivable ground.

The signature of Inca masonry is mortarless ashlar work: the stone blocks are cut so precisely that no mortar or shim fits between them — the faces meet so exactly that a knife blade cannot be slipped in. This jointlessness is not aesthetic but seismic. The Andes are an active earthquake zone; during a quake the blocks, set flexibly rather than locked together, shift slightly, 'dance', and settle back into place when the shaking passes. A mortared wall would crack; the mortarless Inca wall absorbs energy through movement. Doorways and niches are slightly trapezoidal — that batter, too, adds stability against toppling.

The second invention is in the ground. The city sits on a steep granite ridge; the vertical terrain is stepped into terraces (andén) held by stone retaining walls. But the terraces' real genius is invisible: beneath each is a graded fill system — fertile soil on top, layers of sand and crushed stone below; these layers drain the heavy mountain rain, preventing landslides and keeping the buildings' foundations dry. Machu Picchu shows architecture as an integrated engineering that negotiates with terrain, stone with earthquake, and ground with water.