It's important for non-Canadians to realize that Parliment Hill is not the White House or US Senate. Parliment in Canada is a public commons. There is no security at all on the ground of Parliment and the space is routinely used for large scale public protests and demonstrations, less than a couple of dozen yards of Parliment itself. It's a different ball game.
That's not true. You used to be able to drive onto parliament hill, which was great at Christmas to see all the lights. But in the past few years they've stopped all car traffic except cleared vehicles, they've got Ottawa police providing security along with accusations of kickbacks for the service (I can't find the link as Google is flooded with today's stories on the shooting), they have always had security within the buildings themselves (eg: security guards preventing MPs from entering the House for a vote), etc. Sure we let people in to do tours and such, but you can get a tour of the White House too. Besides, what good would it do to assassinate Harper (our Prime Minister)? He's only PM because his party formed government and he's the current head of his party. Kill him and someone else from his party just takes his place - it would be horrible, but it wouldn't stop our country the way it might if another country's head of state were killed.