Candle Clocks

Clepsydras were great for ancient civilizations such as Egypt, Greece or Rome, but in Middle Ages they started not to be so useful. Armies started to besiege cities, castles, etc, so the water inside them was used to drink, and not for clepsydras. Another problem was was that, with clepsydras, you just had to decrease the regular flow of water for time to pass slower (e.g: by introducing a small stick through the hole of the top vessel). Also, the climate of some cities did not allow clepsydras to work. All these problems required a new clock that solved them.
That clock was the candle clock. It consisted of one or more equal candles, and some marks.  Its mechanism was still quite simple; the first candle was lit, and as time passed, its height reduced. Thanks to the marks, you could know the time that had passed. When the first candle burned out, the second one was lit, etc.
But this system also had some problems: you had to be attentive to light the candles, and if a candle went out, the whole system was affected. Anyway, it was pretty good.
It could be something like this: