Rangaaaa, I'm sorry, but this game will never lose it's "full upgrade" setting. At level 16, you are supposed to upgrade yourself fully to defeat the final boss. If you beat level 16 without upgrading, you will just start the level over. Make sure everyone sees this.
Here's how to get to the secret level:
In level 3, in the room where you see a blue orb, go down to the switch and press it. now, a computer room should open nearby. go in and run towards the northeast corner. in the secret room, press the switch. go back to the first room with the weird gray wall above a pool of broccoli. go across the pool with the new drawbridge and defeat every enemy. You'll get to the military base.
Mogadishu is probably the hardest one in Africa, but if you're having trouble, try doing it a couple times and using the money to upgrade the zombies. It will take patience.
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