Malocchio writes: "The Venetian Jews were transferred to the area called the Ghetto from other scattered quarters of the city around 1500, because the city government sought to protect them from persecution (mainly by militant Franciscan monks) by consolidating their community and guarding it (with protective walls and a guarded gate). So the original purpose of the walls was not to enclose or isolate them, but to keep marauders out."

A direct analogy to the word "asylum", which also changed over time to the opposite usage, in "lunatic asylum".

Prisons are loosely based on the concept of the "cities of refuge" where manslaughterers could seek protection from vengeful family members (see Numbers 35:6ff, Deuteronomy 4:41ff and 19:1ff, Joshua 20:1ff).

Surely there is no shortage of words that have undegone sinister shifts of meaning? Ironic use is the first step to thorough subversion, and the process is unstoppable.