Machicolation

Leibourn Caſtle, Kent. A proſpect of the front of that caſtle. Over the gate, we are told, was a machicolation or contrivance, from whence, in caſe of a ſudden attack, great ſtones, boiling water, or melted lead, might be thrown down upon the aſſailants.

Source: wiktionary

There is another feature of this proverbial place which seems to have contributed to the phrase's significance—that is, the Blarney stone. [...] There is one stone, on the very battlement of the castle, forming the outward surface, the machicolation, or opening, through which stones, hot water, melted lead, or anything of that ardent character used, long ago, to be pitched on the heads of people coming after their cattle, and trying to beat in the outer gate. [...] To kiss it, a man should be held by the heels, dipped through the machicolation with his head about a hundred feet or so from the ground, and his life in the hands of a great many jokers, holding on desperately by the battlement and the man, and wishing the latter all manner of mellifluousness and good luck for the remainder of his life!

Source: wiktionary

Another form of machicolation known as slot (or slit) machicolation was formed by leaving an open slot above and before a gate. Box machicolation, which Kennedy refers to as ‘true machicolation’, appears in Early Islamic fortifications. The direct source is clearly Islamic, although box machicolation is also present on pre-Islamic Syrian tower houses.

Source: wiktionary

The main walls were carried to the top of the fourth story, where a capacious machicolation enclosed the tower, on which there is a parapet wall of great thickness, with arches: this was to protect the persons employed over the machicolations.

Source: wiktionary