The Peace Tower is the clock tower on the Centre Block building on Parliament Hill, which is where Parliament meets. There is a memorial room in the tower that includes a book listing all of Canada's war dead, but it's not the same as the tomb of the unknown soldier.
The National War Memorial is just down the street, and that's where the tomb of the unknown soldier is. After he shot the guard at the cenotaph, the gunman had to run probably about 100 meters to get inside the Parliament building.
Thank-you for the correction and amendment. I’m just re- reporting on what they say on CNN, (for those who are allergic), and I cannot vouch for their information, although, I find their round the clock reporting a fascinating way to spend an afternoon laying up with a twisted knee.