The solid widths between the crenels are called merlons. Top of the walls is fortified with a battlement with merlons, crenels and a walkway. The act of adding crenels to a previously unbroken parapet is termed crenellation. This parapet walk, beneath the 16th-century crenels of the curtain wall, connects the three towers. Its crenels were converted into windows with ironwork grilles. Loopholes were made both in the merlons themselves, and under the crenels. Loopholes could be made both in the merlons themselves, and under the crenels. Crenels designed in later eras for use by cannons were also called embrasures. Concrete bunkers were usually only shelters; just a few had crenels.