Twixt = Betwixt or to intertwine or In between from what I understand.
Horizon = a range of your Knowledge on a subject.
Long = Longfellow as in the poet.
Broad = One of any number of female poets OR woman in general.

So my interpretation is that; in between and/or the combination of one's knowledge on Longfellow and "the broad" lies the distance from where the "ground will shake" due to the raising of a draw bridge so that "on one can pass" and when the drawbridge is up you have a deep crevasse.

Any thoughts or knowledge of where a double drawbridge would be that when both sides are raised there would be a deep crevasse in the middle like this crude drawing ___/ \_/ \___ ? There would need to be a double draw bridge to avoid the ring being in the water unless it is from a castle and there is no water in the moat. One problem would be that it would probably be pretty dangerous to be on a drawbridge while both sides are raised.

I have played around with this a bit but I am afraid my knowledge of poetic works and/or draw bridge(s) to be a bit limited.

Appreciate any feedback.

All the best for the new year,
Six
P.S. Broad can also mean obvious...as in broad daylight.