15.1.1 No units may enter a space that contains an unbesieged
enemy fort, either during movement or advance, unless they can
besiege the fort (see also 11.1.8). Units which enter a space that
contains an unbesieged fort may not move nor advance further
during the Action Round.
A Russian corps can't enter Königsberg, unless Königsberg will be besieged after all movement.
11.1.8 Units may enter a space containing an enemy fort if they
are able to besiege it (alone or in conjunction with other units
moving in the same action round) or if the fort is already besieged
15.2.1 To besiege an enemy fort, either at least one
Army or a number of Corps equal to the fort’s LF
must enter the fort’s space during a single Action
Round and then stop movement. For example, to
besiege Verdun, a CP army (full strength or flipped) or three CP
corps (full strength or flipped) would be required. Place besieged
fort markers to help identify that a fort is besieged.