South towards Porto Covo, Odemira
Follow the street along the coast from the Castle. After about two kilometers there is a nice roundabout for example.
North towards Lisbon
It might be worth to walk or hitch (4km from the Castle) to the big roundabout south in direction of Porto Covo. There is a lot of traffic coming through from the south. You will see the sign Lisboa and you can stand under the bridge giving you some shadow. Plenty of cars going fast, but plenty of space to stop as well.