Four flatstones (possibly with some captives, possibly with a cap instead of one of the flats) arranged in a square are called a citadel, and it’s a strong defensive structure. If the opponent captures one corner, not only is it protected by two neighbors, but often such a capture won’t even interrupt the road that goes through the citadel. For example, in the screenshot below, it doesn’t matter if black takes c2 or d3, and d1 and c3 are both well-protected.

However, there are also cases where a citadel can be used offensively to great effect. Behold:

It’s white’s turn to move, but there’s nothing to do — black has won this game. This is perhaps not immediately obvious, so let’s go through some of the options white has.

b2-

White takes b1, black immediately recaptures with 2c1<. The situation remains mostly unchanged.

e1<

White takes d1, but there’s a citadel, so black places at e3 and wins.

e2<

White takes d2, black plays 2d1+, recapturing d2. Black now wants to place on either e2 or e3, which means white can’t block it with a wall, and he can’t take any important pieces.

Se3

White places a wall, black plays d4 and wins

c3>

White takes d3 with his capstone, and this is where the citadel really shines. Black responds with 2d1>, giving us this board:

Let’s go through white’s options again.

b2- is just as useless as before.

2d3- or e1<

White takes d2, black plays on d1 and win

e2-

White takes e1, black plays e2 and wins

Sd1

White places a wall on d1, black plays e1+ and wins

And that’s really it. So, in conclusion, if you can get a citadel right next to an edge, you probably own that edge.

The game that these examples are taken from was between me and ShlktBot, and can be seen in full here.