For our visit to this Tentative site, we decided to visit São João Fortress at the entrance to Guanabara Bay in the heart of Rio de Janeiro. Not that we had much choice in the matter, since most of the locations are further north, and we weren't heading that direction on this trip! The site is still an active military installation, so tours are available via pre-booked appointment only.
First off, it's one of the most beautiful fortresses I've ever visited. Not particularly for the quality of the stonework, the decorations, or anything like that, but simply for the location. It's essentially underneath Mt Sugarloaf (Pao de Azucar), in the rich and trendy Urca neighbourhood, and jutting out into the mouth of Guanabara Bay. It's such a magnificent location, and every moment there felt like a treat.
The tour itself visited a couple of more modern sections of the base (though calling it a military base is a slight misnomer, it's really more of a fitness school for cadets), before heading through a forested area to the historic section of the fort. There's several areas of casemates, bunkers and batteries to explore, though you aren't really allowed to go wandering off on your own. One bunker has a good museum explaining the various phases of history for the fort, and how it was expanded and adapted multiple times to ward off various threats - French settlers, Dutch pirates, English invaders, and so on. There's also good views of the Santa Cruz da Barra Fortress on the other side of the bay entrance; a very imposing defence system for anyone looking to muscle in on Portuguese turf.
Access
As mentioned, the site is a military installation so guided tours are 100% necessary. Check the website first: http://www.ccfex.eb.mil.br/sitio-historico. Tours run at 9am and 2pm, though the tours only seem to run on random days of the week. To make a booking, contact them via email at: sitiohistorico.fsj@gmail.com, check with them which days the tours are operating, then confirm the names and details of everyone in your group. Bookings close a few days in advance - the week we visited, bookings closed on Wednesday and the tours ran Friday/Saturday/Sunday.
Of course, it goes without saying that the entire tour is in Portuguese. The young corporal running our tour did his best to keep us informed via his limited English and Google Translate, though he did mention that we were the first non-Brazilians to have ever gone on the tour!
Logistically, once you've booked a tour it's a very easy site to access. Since we were on the 9am tour we got an Uber from our Copacabana hotel, but realistically any method of getting to nearby Sugarloaf will work fine for reaching the fortress too. The surrounding Urca neighbourhood is very well-heeled and safe - be sure to hit up adjacent Bar Urca for crab cakes and chopp, sit on the wall with the locals and watch the boats motor across the bay.