Non-linear Level Transitions
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Having a linear gameplay is OK but what if you wan't to give the players some choice?
Make a normal level transition (e.g. map 1 to map 2)
Copy it and put it in a different part of the map (a secret room, a different way at a crossroads.)
Make the second transition to either a different map (e.g. map 1 to map 3) or to a different place on the second map (e.g. map 1 to map 2(left path not right))]