This is how you'll survive longer than round 20 on KDT.

Round 1-10

First, you open 1 way to get to the box. Then, buy 2 guns from the box (if they're not good, then buy a gun of the wall, but if they're good just kill the zombies). Then, save up 2500 to buy Juggernog and link up the teleporter.

Round 10-20+

After you get good guns save up 13000+. If you already have Speed-Cola, save up 10000+ to upgrade your weapons. If your guns are already upgraded, save up a lot of money to get new guns, if you want to.

So that's how you get to round 20 and beyond.