Skybridges are not ordinary bridges, but rather bridges that are specifically built to put pressure over the opponents by granting one's self a height advantage by building a staircase to a high level, and bridging from there.
It is a common tactic in OO:SOS, and its not hard to see why. A well guarded skybridge can overwhelm the opponent with the combination of a forward bed.
The way one gets down to the other side of the skybridge is simply jumping into the opponent's water pool, or by laying a water bucket down, which is a form of strategy called Bringing the Rain . However the later option if not executed properly can result in the opposing team climbing the waterfall and potentially overtaking your own skybridge
Skybridging is also in some cases a high risk, high reward option. There is a big possiblity that the opposing team can capture the skybridge and use it to their own advantage.
The way one can capture an opposing skybridge, is while in early process of them building it, one can staircase more higher then they are, this is best done with a sturdy material such as cobblestone. However if the game is being played with a glass roof, then options are largely restricted, one can resort to a Lava Curtain , which is not easily countered when there is a glass roof. The other option is simply putting as much pressure on to the opponent as possbile when they are constructing their bridge, and when they are attempting to go onto the core. In this case archers are a good defense, as there is not much room to hide on a skybridge. Alternatively, if there is enough time one can break the skybridge well enough, but usually if you are in a good enough position to do that, its better to capture the skybridge and use it to your advantage.