The Fortress can be visited
Tour Royale

In Toulon, France, stands the Tour Royale, also known as La Grosse Tour, a 16th-century fortress. Its primary purpose was to protect the entrance to the Petit Rade, the city's naval port. As the first fortification of the harbor, it had early military significance.
History+
The construction of the Tour Royale was commissioned in 1513 by King Louis XII, just 22 years after Provence became part of France. The design came from the Italian engineer Gian-Antonio della Porta, and construction lasted from 1514 to 1524. Due to its size, sixty meters in diameter and seven meters thick, it was named “La Grosse Tour” by the residents of Toulon. Shortly after its completion, the fortress was captured by the troops of Charles V when the commander was bribed to surrender – an event that led to the expression “to be sold like the Grosse Tour” in Toulon. The Tour Royale served various purposes: as a prison for political detainees, as an artillery store during the Franco-Ottoman alliance of 1543, and as a refuge for Protestants during the St. Bartholomew's Day Massacre.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Hervé Rebours
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 43.1027, 5.9258