Just a point of clarification: cannon rush generally refers to the strategy of getting the techs and civics necessary to build cannons long before any of your opponents. You basically abandon balanced technological and social development in favour of getting cannons as early as possible (ideally by around 650 AD), then using civics like slavery to rush as many cannons into deployment as you can. They absolutely chew through any pre-cavalry units. Picture a barely post-Roman infantry going up against 17th century weaponry and you can imagine the carnage. It's not about positioning, it's about getting cannons centuries before anyone else.