If arriving by cruise ship, most piers are within walking distance (10-15 minutes). Just walk north, up the hill, past the Plaza Colon to reach Calle Norzagaray. You will see Castillo San Cristóbal from the Plaza Colon and Castillo San Felipe del Morro is a 15 minute walk up Calle Norzagaray from San Cristóbal. The Pan-American pier is 3 miles away. Taxis are available.
Free trams are available throughout the day and will take you from the cruise ship piers in Old San Juan to both fortifications. Their schedule depends on traffic in the old city, usually a 20-30 minute wait.
Trams stops will be numbered 1 to 26 throughout the city.