Yes , plains shore are like u are searching but forgot to build something that big there if u dont beat bonemass yet , anyway u will do it but no so soon because u need a farm in plains later in game

meadows, dark forest, and plains all 3 have flat land near ocean.
the plains can be troubling as the flying bugs do severe damage to buildings (esp roof tiles and wood bits) and ignore the usual defenses.

you can also just flatten a chunk of land. My harbors are always flattened into the water so I have a place to bust up the ship without losing nails.

Here is something you can think on... I made a very, very large open box (giant U except not rounded) shaped terrain wall and put a stone floor tile 'roof' over a lot of that using ironwood beams. That is the finished basement (the ground is just rock - road type) and port combo, then up the stairs leads into the real castle /fort first floor. Its a Dark forest base, and I cut down a lot of trees and flattened a lot of ground for the work site. The castle part up top is not there yet in this image, its an old/early image.

You can save yourself some trouble: even the extended into the water walls do not stop dwarf swim-around. It takes them a while to get to my boat, but they do get in from time to time.

https://steamcommunity.com/sharedfiles/filedetails/?id=2505240586

just my observation: the plains are typically at the base of or least very near to mountains, so you end up with more hilly terrain making for good hilltop forts. not to say there isn't flat land there, but i feel there is typically far more in the meadows.

Last time I found a super tiny little bit of plains that I could totally engulf I just didn't want to rehash the same seed.

I don't plan on going until I'm ready. I can deal with most things in the plains minus Berserkers without too much issue. Before Bonemass (but swamp Geared) I can deal with up to three Fuling at once in any combo and up to 4 sketters. (As long as I identify stared mobs quickly) so I'm not too worried... I took my first wolves down before I had bronze (I was mining Copper... didn't know about them. I wear the cheese the cheese does not wear me.

My last Castle was 40x20 I think at the inner side of the earth works.

The largest Ring Fort I made thus far has been 4 beams 16 sides... Which gives me a decent internal starting size. But I think I want to go 8 Beams with should make it 4 times bigger internally. Which is about a 36x36 square plus the outer defences so likely about 40x40. It isn't a small space. I don't come across many strategically useful places on the Meadows that will let me build that big almost everything is always sloping up from the beach fairly quickly.

Sounds like I'm going to be going for a boat trip at some point. (I'm long planning... the current seed I put finding the right land as my priority not realize that location I spent hours building in was as far from Black forest as I could have gotten on the map as far as I can see. I ended up building a small outpost and a cart path that is a 3 minute run. So I have time...