Hey Noa_Noa!
Really appreciate the feedback! So, the fill block was kind of an idea (a way of putting rock back after you removed it), but its technically a building, so you have to delete it, and can't mine it.... Its half one thing, half another, and I think I'll probably remove it at some point.
And yes, you can use buildings to block off enemies (currently). Buildings will be attackable very soon, so that strategy won't work at that point.
Again, thanks for following the project! I'll be updating it soon!