It seems people back in the day were always showing of the heads of their dead enemies. did this happen as much as it seems to in movies?
and, as shown in those same movies, did castles maintain permanent spikes just in case a head that needed showing turned up?
Well the practice was far from unknown. The location though varied. You wanted the heads to be seen thus bridges were an option (London bridge had at least 30 at one point) but castles and civil buildings were also used. It also don't always stop at heads. Various bits of William Wallace were displayed around the kingdom
Once in place the heads tended to remain there a long time. This would have made maintaining spikes tricky as they would have tended to fill up. Instead pikes (what kind of castle doesn't have a spear of some type?) or cruder spikes were used to raise the heads above the walls.
That said permanent structures to display those executed have existed. The Gibbet of Montfaucon for example.
Moving onto gibbets I've only ever run across them being made to order and again they could often be in use for a fairly long time (In extreme cases such as those used after the Münster Rebellion over 400 years https://commons.wikimedia.org/wiki/File:Muenster_Lamberti_Koerbe_6428.jpg )
