Krak de Chevaliers.  Just wow!  The inner castle was originally built in 1031 by the Emir of Homs.  The Crusaders reached here in 1099 but it wasn't until 1170 that the massive expansion began under the Hospitallers.  In 1188 Saladin had a crack at the castle (if you'll pardon the pun) but withdrew after only a couple of days. At its peak the castle was home to a garrison of 2,000 soldiers.
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