After buying a map of the local area from Parga, take a bus to the near by village of Anthousa. Ask the bus driver to drop you off at "Ali pasha" (he'll understand that if nothing else). A short walk will take you to the top of the hill where, if you need to, you can buy ice-cream or other refreshments.
The castle or fortress is much in ruins, but it remains a nice place to visit anyway. From the top you can see other villages near Parga, Corfu on a clear day, and the fortress of Parga which Ali Pasha tried to shoot with his cannons. An old lady occasionally sells delicious herbs and spices (such as mint, herbal tea, oregano and different spice blends), so have some small change with you. She's very charming and likes to talk with you but not a word of English.
From the fortress take a walk down to Parga through the orchards and forests of olive trees. The walk is a few kilometres long and walking downhill can be a strain to your knees. The map you bought from the town centre will help you find your way back to Parga.
Alternatively, an easier visit would be to climb the hill aboard the tourist train, which departs every half an hour from the road in front of Krioneri Beach (Parga town centre). The same train will take you back down the hill, and the round-trip will not take more than 3 hours.