A parapet originally meant a defensive mini-wall made of earth or stone that was built to protect soldiers on the roof of a fort or a castle. Now it indicates any low wall along the roof of a building, the edge of a balcony, the side of a bridge, or similar structure.
Crenellations are one of the most recognizable elements of a medieval castle. These upright projections resemble teeth, bared at invaders to prevent their attempted entries and at allies to show the owner's strength. Each upright section is called a merlon or crenel, and they protected defenders from attacks.
Putlog holes or putlock holes are small holes made in the walls of structures to receive the ends of poles (small round logs) or beams, called putlogs or putlocks, to support a scaffolding. Putlog holes may extend through a wall to provide staging on both sides of the wall. Putlog holes in Cardiff castle.
The inner bailey or inner ward of a castle is the strongly fortified enclosure at the heart of a medieval castle. It is protected by the outer ward and, sometimes also a Zwinger, moats, a curtain wall and other outworks. Depending on topography it may also be called an upper bailey or upper ward.
An arrowslit (often also referred to as an arrow loop, loophole or loop hole, and sometimes a balistraria) is a narrow vertical aperture in a fortification through which an archer can launch arrows or a crossbowman can launch bolts.