The 1st book starts with the main character (Paul), a WWI soldier climbing a giant tree in the middle of the battlefield and finding a giants castle. Other episodes include Tik-Toks and Paul on (John Carter's) Mars.

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    The answer below is correct, but just wanted to add that it's not a trilogy but a quadrilogy – System Down Nov 22 '13 at 17:58
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    Tetralogy, to be pedantic, is the only correct term. – James McLeod Nov 24 '13 at 3:16

The Otherland series by Tad Williams :-)

  • Thank you, thank you. Now I can finish reading the dam thing. – laberry Nov 24 '13 at 0:06
  • I wouldn't bother. ;-) Weeell, it's ok, but about 4 times as long as it should be and half the characters could be dropped. – Grimm The Opiner Nov 25 '13 at 9:34

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