It was a semicircular bay, five thousand feet across, which brought the water to Montgomery Street up to the days of ’forty-nine. From a line parallel with what is now Market Street, but a little to the north, was a grand canal, deep and wide enough to accommodate all the commerce of Tlamco. These waters terminated in a basin near the junction of present-day Van Ness Avenue and Market Street, where a circular port of entry was strongly fortified.
Leading from this was a broad avenue, which ended in another circular building, half a mile nearer the Observatory, and in a direct line with it. This was, in modern parlance, a bonded warehouse, and was amply protected.
The port of entry was in the center of a circle which included Telegraph Hill, Lone Mountain and the Potrero hills, all of which were formidable fortifications. Rincon Hill, south and directly opposite Telegraph Hill, guarded the entrance to the canal, while Yerba Buena Island, on the east, lined with its center.
A hostile fleet sailing around Telegraph Hill would be under fire from these forts, and as they came into the canal an assault could be made on them from the ramparts and battlements of Nob Hill.
Should enemies approach the port of entry, they would be in range of the mangonel batteries at East Temple, Alamo Hill, and the Temple of Venus, which also shielded the bonded warehouse and the main, or eastern, avenue to the city. On the top of the hill, was another fortification, guarding the approaches to the Observatory, which had a complete system of defense in itself.
South of the Potrero Hill fort was a harbor for the balsas. It is now a broad marsh intersected by Islais Creek. A curved canal separated two fortified hills and turned west to within the radius of the Camp of Mars. The waterway skirted the closely guarded fortification on Bernal Heights.
From time immemorial Mars was not only considered the god of war but the guardian of sailors as well.
On the west side of the camp, a road ran south, parallel with what is now Valencia Street direct to the port of entry. This was the only approach from the south, and was well protected by the armored hills, where the granaries and storehouses were located.
Much of the food supply came by this route.
Due west of Bernal Heights is a companion hill, which was garrisoned and used as a signal station, being on a line with Mount Olympus, and from the high Observatory tower news could be flashed to all the outlying stations.