Wall mounted caltrop

Well, technically it isn't, as caltrops were usually movable objects, that would present a upward-pointing spike whichever way they were lying. Described as the anti-personnel mines of their day, they were used to great effect for centuries, and even today can be used against wheeled vehicles. They are remembered in particular for their use in the Battle of Bannockburn in 1314, both in the arms and standard of Clan Drummond (whose clan chief, Malcolm, was credited with spreading them to such great effect on the battlefield) and in the naming of streets of modern housing on the outskirts of Stirling close to the presumed site of the battle. There's a whole estate of appropriately named streets, including Caltrop Place, Pike Road and Schiltron Way.
And while it may not be a classic tetrahedron-shaped caltrop, the word comes from the Latin calcitrapa which means "foot-trap" and there's no denying you wouldn't want to stand on it climbing over a wall. A remnant of domestic security from an era before burglar alarms.