Hi, I have been exploring and just finding stuff left right and centre but this one is the biggest one I've found yet. It's not gamebreaking and I am so going to take advantage of this while I can.
However here's Ratbreaths Guide to entering Tortuga City.
As seen above there's a stairway to heaven, this works because the barrier acts like ground, it's a foundation so you can basically climb it using stairs or whatever method you want. Once you reach the top it's only a lil hop onto an invisible ledge and you're walking on thin air like Indiana Jones
Now you want to go to the top right corner of the island where it arches out in a U right behind Tortuga, once there go to the edge of the platform where the deep sea is and just jump...you'll soon find yourself learning how to scooba dive as you clip under the big massive tungston invisible box which has no bottom...so you can kinda just go under it by diving into the water.
After that your'e on the beach and free to explore, build, mine and put bells down.
Hope this report / guide helps!