Buffy the Vampire Slayer Season 4 Episode 21

Mr. Ward: Burn it down, gentleman. Burn it down and salt the earth.
This is a reference to the actions taken by the Roman Empire when they finally defeated the Carthaginian Empire at the end of the Third Punic War, 146 BCE. The city of Carthage was burned to the ground, the navy destroyed in the harbor, and thousands of Carthaginians were sold into slavery. It is a popular legend that salt was worked into the fields so that nothing would ever grow there again, however there is no historical source to support the claim.

Giles: The Trojan Horse
The Trojan Horse is a famous military maneuver from Greek Mythology. It was a large hollow wooden horse used to win the war against the city of Troy. The Greeks declared a truce and delivered the horse to the Trojans as a peace offering, but the Greeks had hidden thirty of their best men inside of it. After being brought into the city and waiting for nightfall the warriors came out of the horse and led a surprise attack against Troy.

Spike: (to Buffy) Look at little Nancy Drew.
Nancy Drew is the title character in a series of books called The Nancy Drew Mystery Stories beginning with The Secret of the Old Clock in 1930, but has spun off into other series. The famous books, aimed at a young teenage audience, are about a girl who "could solve any mystery in 180 pages, with pluck, determination, and sharp wits", as quoted by the official Nancy Drew website.

Spike: (about Willow and Buffy) The little witch gives her the info and pop - Alice heads back down the rabbit hole.
A reference to the 1865 Lewis Carroll book Alice's Adventures in Wonderland, where a day-dreaming Alice spots a hasty rabbit in a waist-coat and follows him down a large rabbit hole into Wonderland.

Buffy: I've never really been one to toe the line.
The expression "toe the line" means to follow the rules or to conform. It calls to mind a row of people the toes of whose shoes are all perfectly lined up, as military personnel at attention or runners preparing to begin a race.

Xander: That's right up there with duck and cover.
In the very early days of the Cold War, "duck and cover" was advice that was given to the general public in case of a nuclear explosion. The government gave this advice (including such ideas as hiding under school desks) strictly as a way to keep people from panicking, because it would have no practical effect whatsoever.