A dueler is only a good as his finest lift.
That doorway looked completely awesome, but I also think that making sand traps would be pretty cool too (Using set of pistons above a thin hallway, then having a button (or other trigger) to release a whole bunch o sand above it.) would be baller as hell.
I reckon that if timed correctly it would be perfectly possible (if it works correctly, that is) to move a minecart to a different track while moving. Or for that matter simply stop one so that you can catch it.
oh yea and check out yogscast survival island videos on youtube they're entertainingly funny