the problem is that you need to read the link i gave you if you understand this you will have no problem
lets say we have 5 cars all the same copies of one car but each one with different stats or look (faster slower color size etc...)
and we want them to behave different to achieve that you need some instance variables for each car
speed
color
size
then you can set each car to behave or look different depending on the values of those variables
so if a turret is level 2 you have to check for a condition so the bullet coming from that turret has longer range or bigger dmg
example
compare turret instance variable "level"
turret "level"=1 bullet set bullet instance variable "dmg" to 5
turret "level"=1 turret set turret instance variable "range" to 5
turret "level"=2 turret set turret instance variable "range" to 10
turret "level"=2 bullet set bullet instance variable "dmg" to 10
this is a very simple example the way you setup events its all up to you i hope this helps you