Wis. Stat. § 66.0717: Lien of special assessment.

A special assessment levied under any authority is a lien on the property against which it is levied on behalf of the municipality levying the assessment or the owner of any certificate, bond or other document issued by the municipality, evidencing ownership of any interest in the special assessment, from the date of the levy, to the same extent as a lien for a tax levied upon real property.