Complex sentence: The fortifications of Ganja so impressed Nader that he decided on a protracted siege of the city so leaving behind a portion of his force around Ganja he set of with the remainder towards Georgia and Armenia in the west, besieging Tiblisi and Yerevan respectively.
Split into simpler sentences: The fortifications of Ganja so impressed Nader that he decided on a protracted siege of the city. Leaving behind a portion of his force around Ganja, he set of with the remainder towards Georgia and Armenia in the west, besieging Tbilisi and Yerevan respectively.