The Butter Box Caponier
Visit to the tourist attraction of Sweden - the part of Visby city wall - The Butter Box Caponier: is one of two caponiers built in the 16th century with the purpose of being able to supply flanking fire in the defence of the wall.
Wikipedia: In this arrangement, the military doctrines of two separate ages met: firearms and crossbows. Crossbowmen guarded the high, inaccessible tower against soldiers with firearms. The introduction of firearms risked making the wall more vulnerable, and hence the caponiers were introduced. They made it possible to provide enfilade cannon fire parallel with the wall. The caponiers had three floors, a low cellar and above two artillery decks.