During his advance on Madrid, Napoleon was blocked on November 30, 1808, by 9,000 Spaniards under General San Juan in the valley of Somosierra in the Sierra de Guadarrama. Because of the rough and uneven terrain, the Spanish forces could not easily be outflanked. Their positions were well-fortified and guarded with artillery. Impatient to proceed towards Madrid, Napoleon ordered his Polish light cavalry escort of some 87 troops, led by Jan Kozietulski, to charge the Spaniards. Despite losing two thirds of their numbers, the Poles succeeded in forcing the defenders to abandon their position.