You can easily visit all the principal sites in downtown Matanzas on foot. Taxis are readily available in Matanzas, and tend to gather around the Plaza de la Vigía and Plaza de la Libertad. A taxi out to the Castillo de San Severino from downtown should cost around CUC$2-CUC$3 each way.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.