* Syrian army advances along Lebanese border * Crusader castle is a UNESCO World Heritage site * Shells land in Leba**n (Recasts, adds quote from academic) By Oliver Holmes BEIRUT, Leba**n, March 20 (Reuters) - Syrian soldiers have raised the national flag on the battlements of a 900-year-old Crusader castle after a three-month siege that ended on Thursday when rebel fighters fled. The recapture of Crac des Chevaliers, a UNESCO World Heritage site, is a real and symbolic victory over divided rebel factions. The army has also wrested back control of most of al-Hosn, the town below the hilltop castle, which was surrounded by Syrian troops for weeks until many rebels moved out on Thursday morning and the army moved in, according to residents.