The name comes with a reason. Colombo Fort, also called Colombo 01, was a fort that was surrounded by stone walls. The little suburb Pettah is known as Pitakotuwa which essentially means outside the Fort.
The original fort, which has an irregular octagon shape, was first built by the Portuguese in the 1500s as means to ensure the security of the commercial point of the country. The location was chosen strategically to ensure a good vantage point to spot any unwanted intruders who may pose a threat to the blooming spice industry. Eventually, the need for commercial demands exceeded and thus, the fort had to be dismantled.