It seems I have developed a reputation in Ireland and the UK about my history mapping, because SK53 had tagged me in a toot about Martello towers. At first, I somewhat brushed it off, but as he probably knew, my interest was eventually triggered and I looked into the matter.
I remember that DeBigC was the first to tell me about Martello towers, but I can’t remember if he used a specific example or just shared his interest.
For those who aren’t familiar: Martello towers were built by the British Empire at a time when they were still ruling the waves, especially around the time of the Napoleonic Wars, when there was a threat of a Napoleonic invasion into British territory. They continued building them in port towns, so mostly along the coast, but not all of them are coastal. Apparently (according to Wikipedia), they are usually round and built in two levels with the guards living in it (i.e. building). They placed a cannon on the top which had a 360° reach, although one hopes that they only pointed it seawards.
The good news was that a lot of them were already mapped on OpenStreetMap, mostly with name=Martello Tower. The bad news (as pointed out in the toot) was that there was no consistent tagging scheme. Most of them were mapped as man_made=tower, and some had some sort of historic tag on them (ruins/ castle/ yes etc), but few were mapped as buildings. So, I came up with historic=martello_tower, skipping the proposal process and documented the tagging scheme in the wiki.
(You might want to debate whether tower:type=watchtower or tower:type=defensive applies, but since a watchtower isn’t necessarily limited to military use, I went with defensive.)
A cannon on the restored martello tower on Mauritius
I first tidied the ones in Ireland and the UK. Judging from OSM, Ireland is the country with the highest density of martello towers, but I’m not sure if that is/ was true at the time when they were still in use.
I also looked at Wikimedia where a lot of them are documented. They seem to have quite a fanbase. I also tidied up the categories on Wikimedia, while I was at it.
This led me to look at the martello towers further abroad like North America, Mauritius, Bermuda, India, Australia etc.
If I remember correctly, the martello tower at Fort Denison was the last one to be constructed.
Not all the ones named “Martello tower” fall under the above definition. There was one example on wikimedia in North Africa which had been built by Governor Haji Sharmarke Ali Saleh, and the photograph didn’t look at all like a martello tower, so I left that one alone.
There are now 92 mapped on OSM (overpass); I don’t know if that is all the surviving ones ore not.
(On a side note, especially to DeBigC - I wonder if all the Irish and UK ones have benchmarks on them!)