Cartagena is a great port to have as part of any Mediterranean itinerary. It is an easy walk off from the ship into a town which offers clean pedestrian streets with plenty of shops, cafes and restaurants. There are a variety of fascinating sites including Roman ruins, Banyan trees and a medieval castle which sits high atop a rock bluff overlooking the harbour. You can spend as much or as little time ashore in Cartagena and will remember it as one of the best days of your cruise.
Cartagena is hard to find in most guide books, but we found two excellent resources to help plan our day. The first is Tom’s Port Guide (https://www.tomsportguides.com/uploads/5/8/5/4/58547429/cartagena-spain-08-20-2013.pdf) which offers detailed directions around town and an excellent walking tour right from the port that covers all of the sites discussed here. The second was the Cartagena Tourism website (https://turismo.cartagena.es/index.asp?idioma=2) detailing the hours and cost of each site (which are very reasonable). The link offered here is to the English translation. The Roman ruins, which are scattered throughout town, deserve an article all on their own so we will discuss those next week. 
From the cruise port it is a short walk to the harbour, lovely waterfront park and promenade. To the left, the tree lined pedestrian main street offers a variety of high end shops, interesting architecture and cozy cafes. The Santa Domingo church dominates the street and is worth the time to explore its well-appointed interior of religious statues and iconography. Take the time to look at the architecture of the buildings as you walk along.
From there turn off the main street to find Plaza San Francisco. The square is rimmed with the most interesting old growth banyan trees. Benches are strategically placed to allow the best people watching.
Further on take the panoramic lift to visit Castillo de la Concepcion, a medieval castle atop a bluff overlooking the harbour and the rest of the city. There is a fine display of period armour and costumes, but the real purpose of your ascent is the view. Climb as high as you are comfortable and take as many pictures as you can. The grounds are also home to families of peacocks. We were there in early spring and got to see the chicks wandering under the watchful eye of their mother.
Our final stop was the Underwater Archeological Museum back at the waterfront. This modern building highlighted the naval history of the area from antiquity on. It offers interactive displays which the children will love, and depending on your interest in marine history will engage adults as well.
We have been to Cartagena twice and have loved our time ashore there. We are sure you will as well.