[22c]

Not the width of cannonball, but perhaps made wide enough such that you could not fire a cannonball from a cannon at the city without having to move across open land.

I'm not sure if that's an urban legend though. I can't find any references to that line of reasoning in historic recounts of Colonel Light's initial plans for Adelaide.

[lostlogin]

I was trying to imagine a park that narrow that ringed a city, I get it now.

[stavros]

Same here, cannonballs are small.

[progre]

The park was designed by Bergholt Stuttley Johnson