Generally, Tangier is a very safe city compared with many places in Europe. The only trouble you may encounter is the persistent touts whom you should ignore, or the con-men ready to fleece you, and you will encounter these almost exclusively in the medina. There are policemen everywhere and you will probably feel safer than at home.
Dressing like a local - as opposed to white shorts, shoes, and a backpack - will help you blend in and get good reception from merchants, who will often quote you actual prices instead of inflated tourist prices. There are lots of expats in this city that speaks Spanish first, then English and then French. A polite no thank you and then simply ignoring touts does get rid of them.
If you are lost in the medina, you can easily find your way out by going uphill souk/English church/Nouvelle Village or down port. Kids and young men may ask you for money to lead you out a couple of dirams, or to the Cafe Central, but if you are asked if you are lost and do not want help, say "Yes, but happily," and usually that gets a laugh and solitude.
Tangier is a safe place for solo women travelers.
