Thinking that this would be the series finale, creator Joss Whedon wrote the scene in the alley at the beginning for old time's sake.
When Buffy ask Giles how many apocalypses they've been through, he replies "At least six, but it feels like a hundred." Depending on what you consider an apocalypse, this is correct. That it 'feels like a hundred' is a fun nod to the fact that this was the hundredth show.
As Buffy gives her final warning before the battle, Spike remarks that it was no "St. Crispin's Day speech," to which Giles says, "We few, we happy few," and Spike continues, "We band of buggered." This is a direct spoof of the St. Crispin's Day Speech from Shakespeare's Henry V, where the title king rallies his troops. It is considered one of the most rousing war speeches ever, from which the famous "Band of Brothers" line is derived.
This is the 100th show of the series.
As mentioned by Joss Whedon in the DVD commentaries for both this episode and Chosen (2003), this episode was originally written to serve as the series finale, and several ideas that were used in the real finale were originally written for this episode.