When the Magdeburg citizens refused to pay a demanded tribute to the emperor, Imperial forces under the command of Johann Tserclaes, Count of Tilly laid siege to the city within a matter of months. The city was besieged from 20 March 1631 and Tilly put his subordinate Imperial Field Marshal , a Catholic convert, in command while he campaigned elsewhere. During fierce fighting, Imperial troops numbering 24,000, roughly the same number as Magdeburg's entire population, conquered several sconces of the city's fortification and Tilly demanded capitulation.