To make a fake wall, make a page, put whatever you want in it, and then, compute.
Then, edit it again, put anything you want, then click on the Back arrow at the top left corner of the screen, (or app, if its windowed) and then, click Save.
These fake walls will appear as real walls but they do not have collision. (they cannot be interacted with) They can be exposed using the 3D view.
Examples:
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(died on the black color)

For hidden walls, you will have to put anything you want to make invisible, then compute, after that, edit the page you put the not yet invisible walls in and rub them out. Click the Back arrow at the top left corner of the screen (or app, if windowed) and press Save.
The walls will be hidden now. can be exposed hardly with 3D view sometimes unless its connected to other things.
Examples:
(smiley face and text are attached with invisible/hidden walls!)
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(the red X was invisible with the blue color)
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(the 2 platforms were drawn in blue and attached with a invisible blue line)