I write historical fiction set in the medieval period and spent a ridiculous amount of time researching topics like this. The candle clock with a nail (comment 18 mentions it) is one of my favorites — you'd stick a nail at a specific height in the wax, and when it burned down to that point the nail would drop onto a metal plate and clang. Monks used these extensively because they had to wake for the canonical hours (Matins, Lauds, etc.) regardless of season.