How can i make those walls you walk through? You know those walls that look like a wall but when you walk up to it you go through it?

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Did you ever dare to dream that you'd one day
stand upon the cross roads between heaven and hell? - Deckard Cain

Well once you have extruded a pathway, switch to surface mode in top view and select the surface which is the one you want to be changed by repetadly pressing "n" key. Once the right surface is selected, edit the GEO setting to 4 instead of the adjoined value of 0. If you pick the 3d preview and you accidentally selected the wrong surface, no biggy, just press "shift+F" to select the other side of the adjoin, and change the value to 4.

I make wide use of this trick in my new and first multiplayer level: MBHG Castle Fight 1 which should be uploaded to massassi.net in the next few days.