. . . and remember that the battle takes place in the remnants of an ancient
stronghold hateful to the Nazgul . . .
As for the second point, it seems that the Wraiths are not so much 'without
form' as simply 'transposed' into the 'wraith-world'. Thus by bringing
Frodo there as well, they improved their own chances while hindering their
enemies'.
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"Time exists,
Just on your wrist,
So don't panic."
Fran Healy
"War is capitalism with the gloves off and many who go to war know it but they go to war because they don't want to be a hero."
Tom Stoppard
"When I was ten, I read fairy stories in secret. Now that I'm fifty I read them openly. When I became a man I put away childish things, including the fear of childishness."
C.S. Lewis