The 4th wall breaks, my guess given the title of the last episode, we are in another layer of reality in the show. Mrs Flood has created her own reality trapping the Doctor in it. So i guess the ending will be a fake out, Mrs Flood thinks she’s won but ah ha! Doccy Who has trapped her in another fake reality within a fake reality or something…& all those destroyed famous landmarks that we saw in the first episode (remember those? weird we haven’t had reminders of that at all) are from a fake reality so not the real earth blowed up.