get towers from the shop(arrow in top right) and place them down in viable places around the map to defend the exit! press the play button to start the exit! THIS WAS THE ORIGINAL BEFORE THE NEW ACCOUNT CAME.
Credit to https://scratch.mit.edu/users/warfame for the original project, go check him out. music from scratch @LordTD24 for being the new one and having this old account to exist.