Cartagena although beautiful does have it's problems. If your staying in the centre don't walk outside the wall after dark. Don't hail a taxi from the street, call one instead. Try to ignore the vendors as they are on every street, it can wear you down saying no all the time.
Beware of pickpockets, during the day walking around the old town is safe. There are many police around and people are willing to help if you get lost. If your looking for a post office to post your cards you won't have much luck. I was advised to go to the Avianca building located just outside the wall heading towards Getsemani , however when l got there they did not sell stamps instead they wanted me to put my cards in one envelope to send to Europe. I was told there was a post office in Getsemani but could not find it.