The desolate, Blitz-like setting was influenced by contemporary reports of fighting in Beirut during the 1982 Lebanon War. Christopher H. Bidmead
also wanted to put the Doctor in a situation where he was very vulnerable, and so he devised both the far-far-future timeframe and the apparent destruction of the TARDIS. See more
Veruna is where one of the last surviving groups of mankind took shelter in the Great... er. Yes. Well, I suppose you've got all that to look forward to, haven't you.
In the Great what, Doctor?
All civilisations have their ups and downs.
[Reading gleefully from the TARDIS log
Fleeing from the imminence of a catastrophic collision with the Sun, refugees from the doomed planet Earth...
Yes, that's enough, Turlough.