The similarities in appearance between The Pandorica and the TARDIS in siege-mode are undeniable, but the connection goes a little deeper; both structures are designed to be impenetrable from without and inescapable from within.
Is it possible that The Pandorica was of Gallifreyan construction, or at least built using Gallifreyan knowledge and stolen Time Lord technology?
I’ve previously speculated in this post that the renegade Time Lord known as The Monk was directly involved in the Pardorica Initiative because he was responsible for the building of Stonehenge, and we discovered in The Pandorica Opens that Stonehenge was just a bloomin’ great 'X marks the spot' for the Pandorica.
Was the Monk an agent for the Alliance? Was the Pandorica built by the Monk, based on siege-mode TARDIS principals?
Of course, the similarities might just be the result of co-incidence and unimaginative writing in which case… meh!
And, just a passing thought: My concept for a siege-mode TARDIS would be sort of egg-shaped, or perhaps resemble a turtle shell; I think either geometry would be far sturdier, more defensive (and more visually interesting on screen) than a cube.