This is the 4462nd most frequent Russian word.
войсками
"By troops" or "with forces" (instrumental plural of "войска," meaning "troops" or "forces").
In this sentence, 'войсками' is instrumental plural, indicating that the troops are the means by which the city is surrounded.
Армия защищала город, окружив его войсками.
The army defended the city by surrounding it with troops.
Here, 'войсками' is instrumental plural, highlighting the troops as the entities being commanded by the strategist.
Стратег руководил войсками во время битвы.
The strategist led the troops during the battle.
In this example, 'войсками' functions as the instrumental plural, showing that the action of crossing the river was performed by the troops.