From the dawn of time, the natural barrier of the Gorge of the Tinazzo was an easily defensible and manned border.
The gorge thus was, for centuries, a strong element of spatial demarcation. At the beginning, it defined the border between the Roman Gaul and the free tribes of the Camuni. During the Roman Empire it served as border between the Volturia and the Quirina tribes. Later on, it divided the Lombard duchies and the Episcopal principalities of Bergamo and Brescia. Today, still it represents the border between the diocese of Brescia and Bergamo and the municipalities of Lovere and Castro.
Along this ancient spatial demarcation, in the XI century, the fortified harbor of Castro and the so-called Road of the Corna were built, the latter built into the rock at the top of the gorge.
Both the village and the road held a high strategical factor: they in fact allowed to connect, through the lake, the plain and the valleys which were subjected to the principalities of Bergamo, without having to trespass into the hostile territory of Lovere, which used to belong to Brescia.
The high Valseriana and the Valle di Scalve were important manufacturers of iron and silver. Still, they depended on the plain and on the towns for food provisioning.
Along the way that from the Po plain leads to Val Camonica through the Val Cavallina, the final course of the Borlezza River and its gorge (the Tinazzo) make a strong natural barrier.
The thick walls of the gorge, right after its beginning, connect together so as to create a natural bridge which, in ancient documents, was called pons terraneus (from which come the name Poltragno, a dainty place nearby). On such a natural bridge many armies passed by, including those of German emperors descending to Italy for their domination wars or for the pope coronation.
Among them there surely were Frederick I, Holy Roman Emperor in 1166, Ludovico il Bavaro in 1327, Charles IV in 1355 and Maximilian of Habsburg in 1516.
The deep gorge of the Tinazzo to the south of the bridge, and the swampy bed and the heady swirls of water of the Borlezza to the north, prevented crossing of the watercourse and the narrow passage could be easily blocked.
The medieval fortification of the Colle di San Lorenzo (the hill of San Lorenzo) was probably built upon an original Roman fort and had the task to monitor and prevent the passing of the Hills and the encirclement of the gorge to wade the river on its wide and below estuary.
The road of the Corna was built along with the fortified harbor of Castro at the beginning of the XI century to create a commercial lake way between the plain of Bergamo and the high Val Seriana, without ever trespassing into the hostile territory of Lovere, which back then used to belong to Brescia.
The road, which was supposed to be travelled by carriages, was built with an outstanding technical know-how, which had to overcome several natural obstacles by building high supporting walls and several difficult excavations into the rock. Wherever the road had the rock has a base, binaries were built to make the way safe, keeping on track the wheels of the means of transport. In its narrowest point, between the rock walls and the cliff, a huge fortification was built. It was made of a door (or port) that, militarily manned, should have protected the territory of Castro from the back.
Today, of such fortification just the two hinges remain which were destined to contain the door jambs