According to the Center City District, more than 279,000 people work in Center City.
With that many people going in and out of the buildings, security is a big focus.
For some, Monday's shooting in New York City makes it an even bigger concern.
At any given time during the day in Center City, a constant stream of people revolve through the doors of high-rise buildings. Many of them are workers in buildings with tight security.
SEE ALSO | 4 dead, including off-duty NYPD police officer, in Midtown Manhattan shooting: officials
"We have to scan in every time we enter the building," said Samantha Schutez of Roxborough.
"They have elevator passes and front guards that seem to be much more proactive," said Tom Sullivan of Moorestown.
Security advisor Maureen Rush says securing buildings in Center City became a bigger focus in the years after 9/11.
As former Vice President of Public Safety at the University of Pennsylvania and current security adviser for the 76ers' new arena, Rush says many of the buildings in Center City operate under a model that uses four layers of security.
"Your outside door is the first perimeter. You get inside you have security in the lobby, the third is the desk. The fourth may be a portal," said Rush.
MORE | Midtown Manhattan shooting victims: What we know about those killed
Even with all those precautions, tragedy can strike.
On Monday, a gunman armed with a high-powered rifle walked into a Manhattan office building. He made it to the 33rd floor as he shot and killed four people, including a security guard and an off-duty police officer.
"When you have someone who is mentally ill and has a purpose and is willing to die for that purpose walks in, it immediately takes everyone off guard," said Rush.
"It's unsettling," said Pennsylvania State Senator Sharif Street.
Street, a Democrat, says the conversation goes beyond just security.
"It just reminds us of the need to have comprehensive reform on the use of weapons of war," said Street.
Ebony Jackson, who works in Center City, says one of her biggest worries is people who have wandered into her building off the streets.
"Sometimes they'll wander in with us and then they'll try to get in the elevator with us," she said, "and we have to tell the person at the front desk that this person doesn't have a badge."
Still, Rush says the security precautions companies take are working to make Center City safer.
"If your building's safe and it's been safe, it's not by accident," she said. "It's because of the measures that you put in place."
Rush added that many of the building owners and managers in Center City communicate with each other, so if a security threat happens in one building, others can be ready to respond.