Complex sentence: During this time, without Swedish knowledge, Polish field marshal Aleksander Gosiewski marched their direction with his army of 4,000 (according to previously captured Lithuanian prisoners) to evict the Swedish troops from the village, by launching a surprise attack.
Split into simpler sentences: During this time, unbeknownst to the Swedes, the Polish field marshal Aleksander Gosiewski approached in their direction with his army of 4,000 (according to previously captured Lithuanian prisoners). Gosiewski's aim was to evict the Swedish troops from the village via a surprise attack.