Short answers, pulled from the story.
The Battle of Paris took place on the 30th-the 31st of March 1814. Allied forces attacked on the 30th, a ceasefire was agreed in the late afternoon, and the French evacuated the city on the 31st of March.
Joseph Bonaparte held overall command of Paris's defense, with Marshals Auguste de Marmont and Edouard Mortier commanding the right and left wings respectively. Joseph's chief-of-staff was General Maurice Mathieu.
Historians give varying figures. Historian George Nafziger estimated the Coalition force at 145,500 men. French strength is given as approximately 25,526 line troops plus around 12,000 National Guards, with only about half of the National Guards armed with muskets.
Napoleon had rejected a fortification plan in January 1814, believing that walling in his own capital would make him appear weak. When the plan was revived, Joseph Bonaparte delayed it pending Napoleon's personal approval, which never came, leaving Paris defended only by 56 wooden barricades and a small number of cannon.
Tsar Alexander I intercepted two uncoded French dispatches: one from Napoleon stating he planned to move toward the Marne, and one from Chief of Police Jean René Savary reporting that Paris's magazines were empty. General Karl Wilhelm von Toll advised advancing on Paris with most of the armies while sending 10,000 cavalry under Wintzingerode after Napoleon as a diversion, and Alexander adopted this plan.
Napoleon abdicated unconditionally in the Treaty of Fontainebleau on the 11th of April 1814. Before his abdication, his marshals refused to continue the war, Marmont's corps of 11,000 men secretly defected to the new French government, and the Senate voted to depose Napoleon and his family on the 2nd of April.