A large river-island in the
Anduin, to the east of the land of Anórien. It was held in defence
against Mordor by Gondor to prevent crossings of the Great River,
though these defences were breached during the War of the
Ring.
The island itself was long and narrow, a little over ten miles
in length. At its northern end, sharp rocks split the waters of the
Anduin in a bubbling foam that gave the island its name. Cair
Andros was covered with trees for much of its
length.
Strategically, the island was of immense importance: apart from
the bridge at Osgiliath, it was the only practical means for an
army from the east to cross the Anduin into Gondor's heartlands.
The Gondorians took this risk seriously. Amon Dîn, the first of the
Beacon-hills, was set up originally to warn the citizens of Minas
Tirith if Cair Andros was ever breached.
Gondor took steps to fortify the island, too, but the history
of these fortifications is difficult to establish. We know that it
was 'manned' at the time of King Ondoher's fall in III 1944, but
whatever defences were in place at that time were apparently
abandoned, since Denethor's great-grandfather Túrin II found it
necessary to fortify the island again in about the year III
2900.
The Gondorian defences fell on 10 March III 3019. On the same
day that the Witch-king rode from Minas Morgul with the armies that
would besiege Minas Tirith, Sauron released a smaller force (though
still numbering more than 6,000 Orcs and Men) from the Morannon.
They overwhelmed the defenders of Cair Andros, and used the island
to cross into Anórien. It was this northern force that blocked the
eastward progress of the Rohirrim as they rode to Gondor's aid,
though after the Battle of the Pelennor, the Rohirrim chased them
down and pushed them back out of Anórien.
They retreated to Cair Andros, and apparently held the island
for some days. In his march on Mordor, Aragorn released a small
group of soldiers from his main army to retake the island if
necessary, and hold it. They must have succeeded, since the last we
hear of the island of Cair Andros, its moorings were used by the
ships of Gondor, as glimpsed by Frodo after completing his
Quest.