From the Estación de Autobuses on Plaça de s'Esplanada at 600 meters from the cruise port you can take a bus to Ciutadella. The express bus takes about 55 minutes, the regular bus 70 minutes. The ride costs € 4.80 per person (one way). In the morning there are 2 buses per hour. At noon there is 1 bus per hour.
The bus stop in Ciutadella (Plaza de los Pinos) is within walking distance of the historic center. You can consult the bus timetable in advance.