IMPRISONING WIZARDS
If you move a tower in such a way that it newly covers wizards, you imprison them until they are uncovered and thus visible again.
When you imprison one or more wizards (even if only your own), you may fill one of your empty potion bottles (no matter how many wizards you imprisoned) by flipping it over.
If you have no empty potion bottles, you can still imprison wizards, but this will not fill a potion bottle for you. Because your imprisoned wizards are covered, and thus hidden, you must try to remember where they are, so you can free them later.
You are not allowed to peek under the towers!
Tip: It is much easier to fill potion bottles earlier in the game, when fewer wizards are imprisoned.