Which building did the conspirators plot to blow up?

The conspirators planned to blow up the Houses of Parliament. More precisely, their target was the House of Lords, where King James I and both Houses of Parliament were due to gather for the State Opening on 5 November 1605.

The plotters hid 36 barrels of gunpowder in an undercroft directly beneath the House of Lords. Guy Fawkes was discovered guarding the explosives during a search late on 4 November, after an anonymous warning letter alerted the authorities.

“Houses of Parliament” is the familiar quiz answer, although it can refer to Parliament’s two chambers rather than one building. The plot was not aimed at Buckingham Palace, St Paul’s Cathedral or the Tower of London. The explosion was intended to kill the king, leading politicians and much of the Protestant ruling establishment.

The medieval Palace of Westminster then housed Parliament and royal courts. The present Palace of Westminster was built largely after a major fire in 1834, so it is not the same structure the conspirators entered.