I've been using Google Earth pretty heavily lately, organizing a massive collection of "Forts of the World" waypoints I found a few years ago.
The main batch wasn't an exhaustive list--only 4 from Japan, 10 on the island of Malta (there are more), only a few from the United States. I've still got some sub-foldered waypoints to look through, though, and I don't know how many more I'll find. The count is up to more than a hundred just with what I've finished so far.
After years of not dealing with the long, tangled mess of waypoints seemingly randomly dumped into a Google Earth "places" file--and after years of just not wanting to bother trying to organize it all, because GE doesn't make it easy--I set up subfolders and got to it. Some of the locations are spectacular, such as four French forts up in the Alps and a pair of Italian forts perched on sheer mountain faces with no way to reach them except switchback trails.
I found Germany's WW2 flak towers in Berlin, Hamburg, and Vienna, lonely desert guardians like Masada in Israel, thickly overgrown batteries like those of Fort Saint Philip on the Mississippi south of New Orleans, fortified cities in the Netherlands, and the "Hermit Fort," Fort Fisher in North Carolina, where an old man named Robert Harrill took up residence in an abandoned WW2 bunker in the 1950's.
There are so many more! Rio de Janeiro, South Africa, Denmark, Sweden, France, Montenegro, Russia, Czechoslovakia, Canada. Makes me wish Mitt Romney would adopt me so I could get an allowance and travel the world just to look at forts and castles (forget about converting to Mormonism, though).
The next part of the project will be adding all of my own placemarks (mostly American forts) and adding new ones for batteries that were added during the late 1900s, WW1 and WW2. Once I've got a honking big batch of them, I'll figure out how to upload the placemarks file and make the world a better place for history buffs.