Complex sentence: On 19 December, government forces approached the Palmyra Castle after a violent battle with ISIS, this latest advance left them within 400 meters of the castle and less than 2km away from the city's gates.
Split into simpler sentences: On 19 December, Syrian government forces approached the Palmyra Castle, after a violent battle with ISIL. This latest advance brought them within 400 meters of the castle, and less than 2km from the city's gates.