The origins of the square go back well before Waterloo, in the English civil was the objective of the pikeman was to protect the muskets with a wall of steel pikes. By forming a square the horse cannot find a flank.
A Horse will not jump a human, it is simply not in its list of tricks, especially as it is faced with a hedge of bayonets.
For a really good example of the rapidity a square can be formed, watch the Four feathers, with Heath Leger. Ok a poor film but a good example of the square in rapid formation and development.
The square was perfected by the latter part of the C19th, especially in the Sudan of 1884/5, Tofrek, Abu Klea, Tamaii, in finally Ondourman, a thoroughly modern army, with modern .303 magazine rifles and machine guns, in a formation that Oliver Cromwell would be proud of.
The square, as tight as it is, in my opinion would not have saved the day at isandlwana, it would have delayed the inevitable. In the words of Kipling,
"for all the odds agianst you, you crumpled up the square.
google "barrack room ballads by kiplin and look up "the Fuzzy Wuzzy" to see the qudos the British army helf the square!