Is this a fortress?

[JustinHorton]

This is a position from an online rapid game a couple of days ago, after White's 36th move. Forum users will be amused to know that I had the Black pieces and found the Worst Move On The Board.

Anyway once I'd got over the shock of that, I started wondering what I should have played in order to secure the win I assumed was there, with my extra bishop for only a pawn. I can't find it though, not on my own, not with the help of a few people on chess Twitter, not with the help of Stockfish. I can crank Black's perceived advantage up as far as +4.5, but I can't find an actual win.

What do you reckon? Is it a fortress or is there a hidden way in?

[Matthew Turner]

Quite some accuracy appears to be required but I think the plan is
Stick the Bishop on e3, bring the King to queenside, then
Be8 a5 b5 (axb5) Bxb5 a4 (bxa4) Bxa4
Then enter with the Black King, either on the queenside or perhaps via f4 if White defends c2. As I say, care is certainly required, but I cannot see White holding out.

[JustinHorton]

OK, this is the sort of thing I've been looking at. (annoyingly I just managed to wipe out most of my Stockfish analysis, but the themes are easily recognised I think). One problem is that I can't see how Black gets in on the queenside if there's a Black bishop on a3-f8: if you manoeuvre the Black bishop round that side then the g5 pawn is liable to fall, as is the d4 pawn if White attempts a kingside invasion.

[Matthew Turner]

One problem is that I can't see how Black gets in on the queenside if there's a Black bishop on a3-f8

If you let me play Bxc2 then I'll come in via the White squares (on the queenside). If you play Kd1 to defend c2 then I'll come in via f4. I don't see how White is defending.

[JustinHorton]

How will you get to f4?

[Ian Thompson]

I think Matthew is right. You can get to a position something like this by liquidating the queenside pawns.

[FEN "5B2/8/8/3pk1p1/b2p2P1/3PbP2/2P1K3/8 w - - 0 24"]

White is in zugzwang and can't prevent both ... Bxc2 and ... Kf4. If White gives up the c2-pawn, then Black can aim for this position.

[FEN "5B2/8/8/1b1p2p1/3p2P1/3PbP2/2k1K3/8 b - - 0 36"]

So long as Black makes sure the d-pawn is always attacked by either bishop or king, I don't think White can stop the black King getting to c2 without losing the d-pawn.

[Matthew Turner]

I think the White Bishop just gets overworked. You cannot stop the King getting to f4 and b4 as you'll get trapped on either a3 f8 diagonal or b8 h2 diagonal

[Kevin Thurlow]

"Forum users will be amused to know that I had the Black pieces and found the Worst Move On The Board."

Oh no, not Kf4...

I benefited from such a move a few months ago. I also saw some online footage of Magnus Carlsen playing a friendly bullet game against a GM assistant and winning like that. Magnus roared with laughter and said, "Where are you going now? Where are you going now? ha ha ha." At least I guess you were spared that.

And in those winning plans, you have to be careful how you play Kf4...

[JustinHorton]

I think the White Bishop just gets overworked. You cannot stop the King getting to f4 and b4 as you'll get trapped on either a3 f8 diagonal or b8 h2 diagonal

I don't think there's an actual sequence of moves to demonstrate this.

[Matthew Turner]

OK, so we'll have
White c2 d3 f3 g4 Kd1
Black d4 d5 g5 Be3 Ba4 Ke6
Where would you like the White Bishop?

[JustinHorton]

On h8 I should think (d8 also appears to be OK). Edit - or anywhere on the a5-d8 diagonal.

[Matthew Turner]

It looks like I've made a mistake, I had assumed that playing Kc5 would be winning, but I now see that I need to get to b4. Back to the drawing board.

[Christopher Kreuzer]

"Forum users will be amused to know that I had the Black pieces and found the Worst Move On The Board."

Oh no, not Kf4...

Oh dear. Is this an important consideration in the fortress question? If the Black king can never go to f4, then it might be impossible to win the position if that is one of the plans needed.

[JustinHorton]

Well it can go to f4 once the White king has been deflected to d1. But the trouble is that by then, it seems that the White dark-squared bishop can prevent the Black king from reaching the square.