For any wargamers who think that you can form a nice, neat square in one turn, this sequence of photographs show what actually happens when something like a half-battalion is approached by a troop of cuirassiers.
http://thomason-photography.net/Waterloo/CavalryAttack2015/
Note how the light company and the Rifles decided to form their own defensive clumps because there simply was no time to safely reach home.
This then gave the rest of us a problem because there was a light company sized hole in the rear of our square!
Incidentally, talking to one of the spectators the next day, he told me that he watched this incident and said to his wife “Look at those chaps – they’re not going to make it.”, which was very much my own feeling (in the square) at the time.