Twenty One Guns were fired from the Castle on account of a very great Victory gained by the Russians Prussians and Austrians over the French in the north of Germany near Leipsic on the 18th and 19th of October.
This refers to the great Battle of Leipzig in which Napoleon was defeated decisively for the first time in battle. The outcome of the battle forced the French commander to retreat back to France. The Allies, acting on the momentum of their victory, invaded France early in 1814. By the spring, Napoleon had been foced to abdicate and was exiled to Elba. Involving over 600,000 troops, Leipzig was the largest battle to be fought in Europe until the First World War.