Is it plausible that in the ancient world there could be a massive gate at the mouth of a harbor that lets ships in and out? One that must be opened and shut with a key (perhaps a key to a gatehouse or a wench)? I was picturing something like the old drawings of the Colossus of Rhodes, but instead of a massive statue there will be a large gate. Most modern harbors are good examples too, but at the mouth between the jetties there would be a large gate.
It would be completely plausable. Do you mean one that swings? One that comes up out of the water would be more likely.
yes, because a swinging gate would have to fight the force of the water and tide, while a rising one simply slides up into it... google the 'thames barrier' for diagrams of that unique method of flood control...
Very interesting. Thanks for the advice, and especially for pointing out the likelihood of a rising gate rather than a swinging one.