Juggling
In Desktop Tower Defense “Juggling” plays a big role in hard modes, it can be even used in Normal mode of the game but it is not that important but if you play harder modes, you gotta use juggling unless you have super duper maze with effective tower placements that basically pwns anything that goes through it, if thats not the case then you gotta juggle, basically it means, opening closed spots so the creeps run to that spot when they reach that spot you close it and open another spot so in practice they are always wondering around the maze because you keep closing and opening different exit spots, it sounds easy but at hard levels it is very hard if you do not use keyboard shortcuts and thing to remember every time you sell a tower, the timer keeps increasing so for example if you sold your first tower in 3 seconds the other one will take 4 seconds, or something like that I am not sure about the exact timing but that is how it works and I think it kinda keeps the game balanced.
Here is a better explanation mate you got it wrong i imagine…
What this involves is make two separate exits that the creeps can go through the maze. When the creeps get close to one exit, you block it, causing the creeps to head to the other exit. When the creeps get close to that exit, you unblock the first one, and block the second one. This allows you to keep the creeps going back and forth through the maze for as long as you want.