Aka Tojikomorihime

From Kokoro no Oasis:

Once upon a time, there was a princess who locked herself up in the top of a little tower, and a young man set on becoming the palace artist. When the King decrees that, "He who can paint a portrait of the 'Locked-up Maiden' will be made the palace artist", Gunjou thinks he's set... but the princess is not willing to come down, and Gunjou is not willing to give up.