I've been on holiday for the past week, and on the long drive back home I spent some time daydreaming on wargaming. We've been listening to Harry Potter and the Deathly Hallows during the drive, and the final battle at Hogwarts got me wondering how I might recreate it on a gaming table.
There's lots of systems one could use, but the easiest would be Hordes of the Things (HOTT) and their typical attacker/defender dynamic plays into the scenario perfectly.
For those who don't know (stop reading if you want to avoid spoilers) at the end of the last book Hogwarts, Harry's castle-like school, is attacked by Lord Voldemort and his Death Eaters in an attempt to stop Harry from finding the Horcrux Voldemort has hidden there (and hopefully kill Harry, thus putting an end to his shenanigans). The students, faculty, and assorted friends of Hogwarts hold off the Death Eaters in hopes that Harry will be able to find the Horcrux and put an end to Voldemort's evil. In The Deathly Hallows the battle is merely the backdrop to Harry's adventure, but the wide variety of characters and creatures involved make in an ideal HOTT scenario to play.
One note though--Harry Potter and Lord Voldemort do not take place in the battle itself; Harry is running around looking for the Horcrux and Voldemort is holding back, expecting Harry to seek him out for the final duel. If you want to include them, make Voldemort a God (to reflect his power) and Harry a Hero on the Hogwarts side. I've also decided that most of the wizards and witches count as Shooters, since in the book most of the spells are short-range and do nothing but remove the target from action (through death, immobility, etc.)
Attacking Army: the Death Eaters
Magician General (Bellatrix LeStrange)
Shooters (Death Eaters)
Beasts (giant spiders)
Defending Army: the residents of Hogwarts
Stronghold: Hogwarts School of Witchcraft and Wizardry
Magician General (Minerva McGonagle)
Shooters (students, faculty, and the Order of the Phoenix)
Blades (animated armor)
Hordes (house elves)
0-1 Behemoth (Grawp and Hagrid, on a single base)
The table should include a forested area to represent the Forbidden Forest, as well as a lake where the mer-people live on the Hogwarts grounds (curiously enough, the lake monster and the mer-people fail to appear in the final book).