How do you make the map idestructable?

[Mouldy75]

Hi, I'm new to map making and i wanted to know how to make the map indestructable ;)

[SilPho]

Unlock it.

Or if you want the more detailed answer, complete mission 25

Edit: Thanks Plasma

[Plasma]

Unlock it.

Or if you want the more detailed answer, complete mission 25 with a silver or gold medal.
Post corrected!