The old part is also known as "Ciudad amurallada" because of the walls surrounding the whole city. Cars are not allowed in its streets so to go around it you must walk or take a car pulled by horses! Beautiful balconies, narrow alleys with its squares and churches in colonial style. I recomend you go for a walk through the old part at night previous to dinner. You'll easily fall in love there (I did!) if you're not already in love.