To all complaining about the amount of lives: it doesn't work that way.
If you click on an enemy, you can see a section that says "mana cost to banish". Next to that is a number. When that monster reaches your home, you lose mana equal to the cost to banish it, then the monster is destroyed (or, if it's a boss, respawned). If a monster takes your mana to zero (or lower), you lose.
Hope that helps.
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