The first time 'round is the longest. He uses it to figure out the rules.
He's solving a puzzle, and he has to win. (The prize is Clara Oswald.)
The basics of any puzzle is, you need to know its rules. Every room resets, easy enough. But the energy loop isn't completely closed, or he couldn't have gotten in.
So what doesn't reset?
What can he alter?
(A plan is forming, in the back of his mind, one that trades a searing, painful death for a do-over - but not until his ready, not until he has rigged the game a little bit in his favor.)
He tries taking objects out of their rooms. Flowers, the painting, chair. A change of clothes. They always go back.
(Those won't do anyway. He needs something that belongs. Something he would always notice is missing.)
He sees a spot on the floor, marked with arrows. Meant to draw him there, so he could get a good look at the monitors.
He takes the stone tile.
He has a brilliant, complicated idea, and it works. Just a zap with the teleporter cables. Uploads the image of the stone into the memory. Now, with the next reset, the castle will believe the stone had always been moved. (Pity he can't do this too much. The teleporter has limited memory and energy, and it won't do to overwrite the image of himself).
Now, he needs somewhere obvious to put it, his stone notepad, somewhere he would always be drawn to.
A fresh grave and a shovel - meant to trap and scare him with the death imagery - he uses to his advantage. He smiles grimly at the thought. He leaves, lets the grave reset, and the stone stays buried. No matter how many times he digs it out. If he breaks it, it reforms.
The theatrics of the castle and his own maudlin curiosity will lead him to the missing stone. And then to the grave. There, he can leave himself any message, carved on the blank slate. One zap and one reset, and the message will be indestructible, maintained by the mechanism of the castle itself.
But what message? What will help speed things along?
He spends the next ten years looking for clues.
When he finds Room 12, it's too late.
As he crawls to the teleporter, his limbs blistering with agony, he is pacing in his TARDIS.
I failed, Clara.
I was so close. That wall is harder than diamond, because it has to be. It is the portal to the outside. It's not part of the castle. Not part of the reset. I just have to find it fast enough, and chip away at it. I'd win.
But I will never know this. Not until it's too late.
I can't make it to the grave. I certainly can't dig up the tile. The only thing I can do, is start the teleport. I'll begin again, with no memory, and no message to guide me.
I won't find Room 12 for a decade. And I'll never leave this place while the universe is still alive.
NO.
No?
Of course.
There is one more thing I can do.
The teleport has saved the image of the tile, and himself as he first arrived. It will also save the image of his body. Or what's left of it, as it burns.
His skull, and maybe a few grains of charred bone, will be written into the memory of the castle, un-erasable.
Will that overload the memory?
Sure. After a few thousand cycles perhaps, the old bodies will start to disappear. But he will renew the message, with every cycle.
As he feels his Timelord cells finally give up their fight, and begin to disintegrate, he gathers the last of his strength, and writes.
oOo
The next cycle, he doesn't see it.
Instead, he remembers it, as he stumbles towards the teleport in agony. Remembering in despair how he had puzzled over the blank slate. Realizing with terror and frustration that he will remember his ordeal, every one, but always, always, too late.
But there is hope yet.
The pile of dust that was his body becomes deeper, with every cycle.
oOo
It is seven thousand years into the future, by the stars.
A dying man had used the last of his energy to write I AM IN 12 in the dust of his body. And surrounded the words with an octagon.
He's figured out how the teleport could store memory (For the 3021st time, but he doesn't know that. Yet.)
He considers the blank, octagonal stone slate he's holding in his hands...
oOo
He laughs as he crawls.
Seven thousand years, Clara! Seven thousand years, for me to get the message! Am I a thick old fool or what?
Now, a different message, for the dust. Something that will remind me to strike. Something to make sure I will never fail to chip away at the diamond mountain.
He lifts a trembling arm, and writes-
BIRD