Who knows two hundred thirteen?
Please cite/link your sources, if possible. At some point at least twenty-four hours from now, I will:
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Go on to the next number.
Building on Gershon's answer:
The gematria of the names סיחון עוג is equal to that of כוזו במוכסז כוזו - both 213. The connection: we write these names on the outside of the mezuzah to prevent evil forces (kelipah) from entering the house. Conversely, these two kings were like an "anti-mezuzah" preventing the forces of good (the Jews) from entering Eretz Yisrael, and so they had to be removed first.