Also, is でなくば another form of でなければ?
Apparently something something happened and the speaker apologizes (warui) for indirectly ruining (mizu wo sasu you de) the mood (kandou). I do not know the fuller context, but I interpret the sentence to mean something like:
Sorry to throw cold water on the mood, but hurry up and get on (ride) Niko. If you don't, you'll be crushed along with the castle.