You can go all steps back from what you have created by tapping on the "Undo" button on the top right of the screen as long as you don't leave the level you've been working on. Every time you exit a room or a floor, all your changes and adjustments are being saved.  Basically, the magicplan undo-feature works just like the one in a lot of other programs - you can go back only if your changes have not been saved yet.

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