was then known.

The fortresses give a high idea of the progress made by the ancient
Peruvians in architectural art. These structures were surrounded by
ramparts and trenches. The larger ones were protected by the solidity of
the walls, and the smaller ones by difficulty of access. The approaches
to them were chiefly subterraneous; and thereby, they were enabled to
maintain secret communication with the palaces and temples in their
neighborhood. The subterraneous communications were carefully
constructed; they were of the height of a man, and in general from three
to four feet broad. In some parts they contract suddenly in width, and
the walls on each side are built with sharp pointed stones, so that
there is no getting between them, except by a lateral movement. In other
parts they occasionally become so low, that it is impossible to advance,
except by creeping on all fours. Every circumstance had been made a
subject of strict calculation; it had been well considered how treasures
might be removed from the palaces and temples to the fortresses, and
placed securely beyond the reach of an enemy, for in the rear of every
narrow pass there were ample spaces for soldiers, who might dispute the
advance of a whole army. Besides the remains of the fortress of Cuzco,
which are gradually disappearing every year, the most important are
those of Calcahilares and Huillcahuaman. Less interesting, though still
very curious, are the ruins of Chimu-canchu in Manische, near Truxillo,
which are not of stone but of brick. The architecture of the small
fortress of _Huichay_, two leagues from Tarma, which defended the
entrance to that valley, is very remarkable. The front is built of small
but firmly united stones, and covers a large cavity, in which there are
numerous divisions, intended for the preservation of warlike stores, and
for quartering soldiers. On the steep declivity of the hill there had
been a deep trench, between which there was a wall fourteen feet higher,
flanked by three bastions. Around this fortress nitre is found in great
abundance. It is now collected by the Huancas (the inhabitants of the
valley of Jauja), for making gunpowder. The diggings for nitre have
almost obliterated the entrance to the cavity, and the fortress is
already so much injured that possibly in another century scarcely a
trace of the edifice will remain. Notwithstanding a search of several
days, I did not succeed in discovering the mouth of the cavity,