Nice touches: the "Celtic"; the "you will be remembered" (by anyone who's studied the Cambridge Latin Course); the idea of lifting the characters from the Cambridge Latin Course - as far as I recall Quintus is the only one known to survive, we are just left to infer the fate of Caecilius and Metella.
Weaknesses: Shouldn't Quintus be off in Britannia in six months' time, rather than studying to be a doctor? And couldn't we try a purely historical episode? Yes, it was clever to have aliens preventing history from taking its course, but they were one of the weaker points (at least they pointed out the water content of the Earth at one point). If you are going to have monsters that are so reminiscent of the classic series (I'm thinking Hand of Fear, of course), perhaps you should just say that's who they are. And of course Donna's reaction was very obviously cloned from Barbara's in The Aztecs. And the bit where Lucius et al. were climbing the volcano was so obviously not Italy.
But that's nit-picking. In many ways it was certainly better than the second series of Rome.
Two planets destroyed in two weeks; two references to the Shadow Proclamations. Rose appearing to cross the void that was closed; is it all too obvious as a lead to where this season is going?
And next week, Ood.