I don't have much to go on but...
I watched this some time between 1998 - 2003. It was a TV miniseries (maybe 8-12 eps I think), about a boy (age 11/12 I think) that gets transported to a magical world.
At some point he pairs up with an older (or possibly just taller) girl, who is from that world.
At some point they go to a castle that has big colourful creatures of some kind. In one of the later episodes they go across a wooden bridge over a chasm.
I watched it in the UK, I can't recall if it was UK, Australian or USA but definitely in English.
Unfortunately that's all I can remember!