This is the Peace Tower located in the middle of the Centre Block of Parliament. As its name refers, the tower is symbolic of Canada's many peace missions and was built to honour those Canadian men and women who lost their lives in wartime missions, specifically World War I. The Tower itself is made of Nepean sandstone (a local find), which is durable yet easy to work with and stands a whopping 92.2 metres (302 ft. 6 in.).
FYI: Each morning during the summer, you can stop by for a rich musical experience: an hour-long 53-bell carillion concert in the Peace Tower played by Dr. Andrea McCrady.