Add the wine, cider, cinnamon, star anise, and cloves to a large pot. Bring to bare simmer over low heat, without allowing it to come to a boil, and cook for 20 minutes.

Zest the oranges with a peeler (gently peeling only the bright orange skin). Remove the mulled wine from the heat and add the orange zest, port, and orange slices if you’d like.