How do I make a wall that both heroes & NPCs can't pass?
That's pretty easy! You use the function called wallmap. Under custom, edit your game. Then, edit the map on which you want to put the wall that both npcs and hereos can't pass. Choose "place wallmap". Place the wall with the spacebar.
This method is simple and perfectly adquate when the heroes and the Npcs are in small rooms. Like, a Inn chamber, or a castle romm but may not be appropriate when they're outside like in a town alley in front of a shop or near castle walls.
In this situation you want only your NPCs to be stuck into a certain part of your map so that when the player comes near the location, he can find the NPC he/she is supposed to talk to. To do so, you need to learn how to create zone movement (see the link below for more details)