Portal

Location
0-1 in normal dungeon

1-2 in nightmare dungeon

You can find it randomly or by using a Scroll of Portal Location, it reveals the portal's location on your map.

Portal on map
On the map you can notice a narrow purple line.



Here is close-up of what the portal looks like on the default map:



Blocks

 * - main build
 * - dungeon
 * - dungeon background