Using tiles to stack (via LCtrl + click) after raising up the grid to the top of the previous tile i created a wall that I discovered the NPC and PC could easily pass through. Is there a way to change a setting to prevent this or we have to make sure the ground floor tile where the wall will be built upon has to have a 'pending' navigation declared along the path of the wall to prevent characters walking through it?

*Solved*

I went ahead and experimented. When you place an object or high tile block it takes up 4 grid tiles right? with it being in the center. So if you want a tile object for a wall or column not to be passable you need to declare all four directions or sides of that 4 tile radius as non walkable? this is what i found experimenting. I think i will leave this post for other newbies like myself.

I wasnt able to change the connected tiles after placing the wall though, so i will have to delete them and start again now i know how to do it properly.

Yes, I believe what you have stated is correct. Basically, for a given tile, there are 4 "paths" that can be taken - north, south, west, and east. So each tile stores whether or not you can walk/interact to each of the 4 tiles located in those directions. So, if you don't want characters to be able to walk onto a tile, there should be no navigation lines going into that tile (or potentially an orange "Interact Only" if there's an object on that tile that can be interacted with from an adjacent tile).