Fixed: The path wasn't followed properly (ball wouldn't pass left of tower but through tower)
Fixed: Balls wouldn't disappear at the end of the playing field
Fixed: Fish eye distortion of the playing field
Fixed: There was no check for whether there was a path for the current ball
Added:
   - Towers are firing at the nearest ball
   - Towers have a firing rate and cooldown time
   - Impact of bullets is checked against floor and balls

Todo: more intelligent way to determine ball position relative to playing field grid
Todo: towers need to fire at expected position rather than current
Todo: hit points for balls missing (hit condition present)