This brick fortress flanked by four square towers (1357-1368) was surrounded by a perimeter wall in 1385 and converted into a prison in the 15C. Grotius was imprisoned here in 1619 before managing to escape two years later in a box which had been used to bring him his books. The famous box can be seen as part of the visit, which also recalls his life. The rest of the site re-creates the atmosphere and life of a military fortress which was once part of the Dutch Water Line, a role officially held by Loevestein until 1951.