When I took over The Castle I was kind of frustrated with the giant holes in the walls on two ends of it. I quickly had problems trying to fit in regular walls, then I found a solution that might be of use to the rest of you. The problem with walls is that they don't go through things, but the floors do. The floor foundation under the wood section can be raised, lowered, and it goes through all kinds of objects that can't be moved. It just fits in really nicely as a wall because of its texture, and it doesn't get blocked by objects. Same thing with bridges.

If you have holes you need to fill, like the stonework near the river in Sanctuary, floor foundation works really well to patch things in since it doesn't get stuck.

the floors don't go through everything

I have big problems making a believeable settlement in the Hangman's Alley, because of all the buildings around

They kind of do. If you can't get it to fit, try building one next to it so that it latches onto it. It is hard to explain, like if you have a spot you want it build one just one unit over, then build another where you want it. It works a lot of times.

Yeah I had the same problem with hangman's ally, there's no space to work with & you can't scrap the existing shacks, so annoying. My only option was to build upwards so I get the floor/ground space to myself. Although one of the settlers I assigned to guard duty won't go to his position, maybe because he has to go up 2 flights of stairs to get there.

Some settlements need to be resized. Some are just stupidly sized. Theres one that has all kind of clutter just out side of the building area. The could remove most and leave like 5 or so and I would be happy.

Not saying HA is on a top 10 list for best plots to build on, but given location to DC, worth building somewhat. All shacks can be completely torn down except the one raised up just to right of one doorway.

Build a stair to one of the sides - the perfectly flat square above where the default craft bench is makes a perfect building point. Get on top of that and you can lay down perfectly flat floors, that then snap into place and extend over the entire Hangman's Alley. I built a 3 story + rooftop BBQ and night club structure that basically sits over almost entire plot of HA, with the bottom ground where dirt is available to put down pumps and crops.

Because the structure blocks all above the ground floor, i just placed turrets inward facing from all corners and sides of ground area and it insta wipes whatever random spawn or settlement attack.

The only issue with HA is i dont like the arbitrary height cutoff. It won't let you build multistory above a certain height, which doesnt even reach the top of the side buildings around it (was hoping to build a rooftop escape as alternate to two side doors but no luck due to this restriction.

If only there was a console command to give you the ability to place objects anywhere, even though existing structures. Within your settlement of course.