So, I'm mapping (top down/battlemap style) a stronghold on the elemental plane of earth. Most of this is effectively a dungeon, but one of the things that is unusual is that many of the doorways or connections between rooms and hallways are actually solid stone walls. It requires that anyone passing through either having an ability to burrow or pass through solid stone (effectively keeping out pesky adventurers since earth elementals can pass through solid stone). These passages/portals are different from the rest of the stone walls, as those have been magically treated to prevent passage through them.

So, what look can I do on my map to make these portals/passageways stand out on the map? Maybe some sort of window symbol made stone colored?

Any or examples would be greatly appreciated. Thanks.