Castiel is an angel of the lord or a baby in a trenchcoat. Sometimes both. His eyes glisten with sadness and honesty, wonder and amazement in the darkest shade of sunset blue. So when I compare him to the Tardis, I do it not only as an allegory for the colour of his eyes, but also with all the sadness and joy and bliss the Tardis has seen. Reflected through Castiel as a lost angel, waiting for his Dean.