Well, there's a way - bridging vehicles exist, and make bridges from 'one side':

Youtube video of a 'real' one'

And a LEGO one

It's pretty low tech - but moderately high engineering, as you need a folding span half the 'width' of your bridge, and a rather significant 'ballast' to stop toppling. On a quarter-mile span, that's going to be a rather impressive sort of a challenge - you'll need a bridge 'doodad' that's 2 lots of 200m, which isn't a trivial sort of a number. And enough 'counterbalance' on the bridging vehicle - you'd probably need a ballast on a lever, sort of similar to what you'd get on a gantry crane

But in a world where there's magic, it may be altogether more doable.

Well, there's a way - bridging vehicles exist, and make bridges from 'one side':

Youtube video of a 'real' one'

And a LEGO one

It's pretty low tech - but moderately high engineering, as you need a folding span half the 'width' of your bridge, and a rather significant 'ballast' to stop toppling. On a quarter-mile span, that's going to be a rather impressive sort of a challenge - you'll need a bridge 'doodad' that's 2 lots of 200m, which isn't a trivial sort of a number. And enough 'counterbalance' on the bridging vehicle - you'd probably need a ballast on a lever, sort of similar to what you'd get on a gantry crane

But in a world where there's magic, it may be altogether more doable.

The bonus here is that you could - potentially - recover your bridge, and have a pretty secure fortress....