The Castle Revamp
If you want to change the look of The Castle settlement but don't want to put the hard work in, take a look at this Fallout 4 mod.
The Castle Revamp is a settlement overhaul that turns the massive settlement into a much better looking massive settlement. It adds tons of grass and trees.
It also expands the settlement boundry, gets rid of some of the holes in the walls, and deletes the clutter on The Castle's grounds.
It's an excellent take on the Minutemen's base and one to consider if you continue to find yourself in The Castle when you play.