The Panama canal is of course sited where it is to take advantage of the short distance (around 50km) between the Pacific and the Caribbean. But long before the canal, there was an important land route across the isthmus and it was the access to this that these fortifications were built to protect.
The Spanish used this land route from the mid-sixteenth century to transport their ill-gotten gains (gold and silver in particular) from South America. The goods were brought up the Pacific coast and taken overland to the Caribbean ports where they were loaded onto ships for Spain.
I visited the fortifications at Portobelo, taking a day trip via a couple of buses out of Panama City. According to one of the information boards around this small town, at one point a third of the world’s gold shipments went through here, held in the large customs house to await the once a year arrival of Spanish ships.
Of course, such booty attracted pirates and privateers necessitating the construction and subsequent improvement of the harbour fortifications in the seventeenth and eighteenth centuries. One of those who came to raid was Sir Frances Drake, who died of dysentery while anchored off Portobelo in 1596 and was buried at sea in a lead coffin somewhere nearby.
As with previous sea fortifications, this one was not an easy Lego build. I have attempted to render part of this view:
with this model: