In original DROD secret rooms are rooms which are unrequired but essential to master a hold, meaning that in order to conquer the hold, these rooms can be left uncleared. Secret rooms are distinct by their pink colour on the mini-map, and the flashing message which appears when one is first entered.
Access
Usually, secret rooms are accessed via secret or cracked walls, although this is not always the case - in some holds, there are alternative methods such as alternative solutions to rooms, or scripting.
Restoration
Restoring to secret rooms, unlike other rooms, does not affect progress in the hold. For example, if one restores back to a secret room, completes it, and then continues from the point they were already at, the secret room will remain completed. (needs revision)
Unrequired Rooms
There are some rooms in DROD that are neither secret, nor required, these rooms first appeared in Journey to Rooted Hold, and were used after a blue door had been dropped to essentially make the levels longer. These also appear pink on the mini-map but do not show a flashing message when entered.