Heroes of Loot

Wells appear relatively frequently (probably every 5-6 levels) but in order to use one, you will first need to purchase a Cup of Life. These are sold in stores for 399 coins. The store items are random each dungeon so it is worth checking each one. 



If you have enough money, purchase the Cup then when you next come across a Well, stand in front of it. A bar will now appear and your health will be refilled. Once you have a Cup, you can use it for the rest of the game (you lose it once you die) but you can only use the Well once per level so I recommend waiting until you have cleared the enemies then you can refill your health before moving on.

Unfortunately, as the Cup appears at random, it is possible that you will need several playthroughs to spawn it and have enough money to purchase it. I did not come across the cup as a random drop nor in a secret room.