The threshold represents the sanctity of the home. To some creatures, it is inviolable - others, it merely weakens. Without a threshold, no protection can be anything but temporary - a circle drawn in the sand. An apartment isn't great for it - you really need a home to get a powerful threshold, and it's not a thing that happens overnight. Closest you're going to get there is going to be a latticework of spells and enchantments, one that you have to rebuild every single day when the sunrise breaks it. There is no free lunch, even in magic.
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The threshold represents the sanctity of the home. To some creatures, it is inviolable - others, it merely weakens. Without a threshold, no protection can be anything but temporary - a circle drawn in the sand. An apartment isn't great for it - you really need a home to get a powerful threshold, and it's not a thing that happens overnight. Closest you're going to get there is going to be a latticework of spells and enchantments, one that you have to rebuild every single day when the sunrise breaks it. There is no free lunch, even in magic.