The end of series 6 answers many questions, although the questions were from this particular series itself. Therefore the audience is given a semi-satisfaction of their curiosity.
The alternate reality created when the Doctor was not slain is very entertaining. It is a colourful mixture of both science-fiction and steam-punk, with steam locomotives in use, hot air balloon cars, and many personalities of history existing simultaneously. It was rather amusing to have Churchill as "Holy Roman Emperor", and the Silurian scientist from the previous series as his personal physician. Also, a pleasant cameo from Charles Dickens who returns briefly long after his initial appearance in series 1.
The vigilanteTeselecta shapeshifting robot and its miniaturized crew make another appearance posing as a member of the Silence; they assist the Doctor in his tracking of his unknown enemies. The Teselecta also serves as an integral part of the plot, as the Doctor's on-screen demise at the beginning of series 6 was revealed to be the robot vessel piloted by its crew and a miniaturized Doctor. This manages to technicallyfulfil the fixed point in time of the Doctor's death, which is somewhat insulting to the audience but is an acceptable method for the Doctor to survive once again.
The creepy Madame Kovarian also appears, although this time as a prisoner of the anti-Silence movement lead by Amy and River. She continues to uphold her threatening visage even when captive. Kovarian shows a strong disdain for the Doctor surviving, and is almost comically repulsed by the relationship between the Doctor and River Song; this shows the character's consistency of being opposed to the Doctor in every way, even his personal connections. It is revealed that Kovarian was merely another agent of the Silence, and was not as important in the order as she had believed due to the fact that the Silence kill her just like their enemies.
The enigmatic Silence remain just as large a mystery as they always were, which is both a strength and a weakness. The strength is that the creatures continue to hold the same aura of menace as they did in the series two-part opening. The weakness is that it is frustrating that once again the audience is still left in the unknown.
A darker side of Amy Pond was shown just prior to Kovarian's demise; it was already established that she was despondent over having her child taken from her, therefore it was not completely out-of-character for Amy to not only ignore the Madame's pleas for help when she realised her expendability but also ensure that the fatal electrocution from the "eye drive" was delivered by pushing it into place.
A welcome in-universe acknowledgement of the unnaturalness of the Doctor and River's relationship is shown when Amy, while conversing with a later version of her daughter Melody, realises that after the eponymous wedding she is the Doctor's mother-in-law. It is amusing to see her reaction; although it is not made clear whether she is shocked about being literal family to the Doctor now, about feeling old due to her daughter being married, or any other emotions.
There were other returning and new characters which I may have omitted a mention, but for in general the supporting cast maintained the emotional and story-driven momentum of the episode.
The question that when answered will cause "SILENCE TO FALL", is revealed to be nothing but the show's title itself. I feel rather foolish not noticing the "oldest" question earlier, as I am a fan of this show therefore making my ignorance more painful. Thus, the oldest question in the "whoniverse" is indeed "Doctor Who?" a phrase repeated at least once to every incarnation of the Doctor from First to Eleventh.
This episode was a strong finale for an overall great series, even though many questions remain as frustratinglyunanswered as they were in the last series.