Fall of Cardolan
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A direct assault on Cardolan would probably result in disaster as Arthedain would have probably issued from the Weather Hills and struck Angmar's army in the rear or flank.
Therefore, Angmar was forced to launch an all out assault against the remnants of Arnor, using several forces, one to besiege Rivendell to prevent it from sending aid to Arnor, and through blocking Rivendell, also blocking reinforcements from Lórien. An army to tie up Arthedain along the Weather Hills and Amon Sul, and a final army to destroy Cardolan.
Angmar's attack was highly successful. Rivendell was successully besieged, and Arthedain was hard pressed along the length of the hills. Not only that, but Amon Sûl was taken and sacked despite the heavy resistance. With Arthedain's forces pinned down or routing towards Fornost, Angmar's last army moved into Cardolan, which was a wide and open land, with little to no natural barriers.
It is probable that the Prince of Cardolan gathered what men he could quickly, rather than mustering his entire army, so Angmar's advance could be stopped and Cardolan's agricultural needs would not be laid waste. The Prince attacked, failed and was killed in the process. However, he did enough damage to Angmar so that it lacked the strength to capture Tyrn Gorthad, Cardolan's capital in the Barrow-downs.
Overall, the 1409 Campaign was a huge success for Angmar. No longer was Arnor an unbreakable wall of defences. Although the siege of Rivendell was broken and Arthedain routed Angmar with the help of Lindon, Angmar had destroyed Amon Sül, Arthedain's chief defence in the Weather Hills, and Cardolan was on the verge of destruction and highly vunerable.