The move space ability
A sentry gun unit (tesla if dealing with crawlers)
The bulldozer unlocked with the 50% off ability
the ability to spend 3400 cash at the start of the game
OPTIMISED SSCENARIO:
fully upgraded double sentry guns
the ability to get a triple teir space
the electrical engineer commander

First you MUST be on a map with a bulldozer
Next buy a the bulldozer and a sheriff and pause the time
Put a space in the path of the bulldozer with the move space ability
Now make the sheriff move to that space but have the buldozer move ontop of the space before he gets there
This will make the sheriff invincible
Afterwards put another space next to him and put a sentry tower there, a tesla tower would be best
Finally build up your fortress with more spots and more sentries and you can replace the sheriff with an electrical engineer unit in the later rounds to buff the sentries even more.

Thats it! now enjoy as the cash keeps flowing in. I hope this helped even thoug the only people stil playing this have either not found tf2 yet or people who sunk a bunch of money into it.