Different levels of structures' strength?

[Leech]

Line your wall with obsidian !

Then they dig under

Fill the area under the wall with lava!

[Cell]

Line your wall with obsidian !

Then they dig under

Line the ground with obsidian !

[DjFuzz]

Line your wall with obsidian !

Then they dig under

Line the ground with obsidian !

[Cerce]

Line your wall with obsidian !

Then they dig under

Well, if you look at things historically, walls didn't just sit on top of the land. A lot of them went under the land as well.

Also, the proper term for digging under a wall to either cause the wall damage or to get to the other side is 'sapping'.

[Sebiale]

Well Guys, People will just dig under the walls and fortifications.

I already covered this in the OP--you give the ground beneath a town block protection to a certain depth from anyone who isn't a town inhabitant.
The greatest threat in that situation are infiltrators and traitors. Traitors would need some really heavy RP justification.

[Crashlander04]

It's a unique idea to say the least, but are we really going to modify specific block mining time for each layer?

Overall, I think 100% protection during times of peace is perfectly fine - I mean, we want protection for a reason right?

[Leech]

Line your wall with obsidian !

Then they dig under

Well, if you look at things historically, walls didn't just sit on top of the land. A lot of them went under the land as well.

Also, the proper term for digging under a wall to either cause the wall damage or to get to the other side is 'sapping'.

Spy sapping my wall!

[Android]

Line your wall with obsidian !

Then they dig under

Well, if you look at things historically,

But you have a C in history.