Without doubt, Cartagena's old city is its principal attraction, particularly the inner walled town, consisting of the historical districts of El Centro and San Diego. It is a real gem of colonial architecture, packed with churches, monasteries, plazas, palaces and mansions with their overhanging balconies and shady patios.
The old town is surrounded by Las Murallas, the thick walls built to protect it against enemies. Construction began towards the end of the 16th century, after the attack by Francis Drake; until that time Cartagena was almost completely unprotected. The project took two centuries to complete due to repeated damage from both storms and pirate attacks. Only in 1796 was it finally finished, just 25 years before the Spaniards were eventually expelled.
Las Murallas are an outstanding piece of military engineering preserved in remarkably good shape, except for a part of the walls facing La Matuna, which were unfortunately demolished by 'progressive' city authorities in the mid-20th century.
The best approach to experiencing the Old Town is to wander leisurely, savoring the architectural details, street life and local snacks along the way. Don't just seek out the main sights - there are many interesting places which you will find while walking around.