mapnik - Tilemill

Cartaro saves its geometries in a PostGIS geometry column. You could therefore configure Tilemill to access the Postgres database to read the geometries from there. See Working with PostGIS in Tilemill's guide for configuration instructions.

Within Cartaro you would need to set up a tile layer to access Tilemill's tiles.

Note that depending on the connection parameter you provide to Tilemill, it might have access to more data than what you want your ordinary users to see. You need to check permissions manually in the SQL query provided to Tilemill.

Cartaro uses GeoServer instead of Mapnik because it does provide a wider range of services. Should you only require your map as tiles without fine permissions checks, feel free to use Tilemill as renderer, though.