Portobelo, is an interesting blend of history and culture. However, it would not be advisable to walk alone at night. Panama generally has lower crime rates than some neighboring countries but instances of petty theft do exist. At night, the streets become quite deserted, which might escalate the risk. Therefore, it is somewhat better to explore the city in a group, especially at night or you could hire a local guide to ensure safety. It's always a good idea to remain cautious and aware of your surroundings.