1. ## Detecting fortresses

Here are a couple of silly positions. The first one is a dead position of the sort that a watching arbiter must immediately declare drawn.

 FEN Viewer

The second is dead only from one side. If Black just shuffles pieces white can't do anything.

 FEN Viewer

When I enter the top one into Stockfish 4 it recognises the position is drawn in about a minute.

I have had the second one in for 15 minutes and it is yet to recognise the position is drawn. It gives +1.9 for white.

I'd be interested to know if any better software out there recognises the second position as a draw with best play.

2. It is the simplicity of the first position rather than the fact of it being blocked that causes the computer to find the draw quickly. Because the bishop is immobile it can find the draw by getting to 100-ply. In contrast for this position:

 FEN Viewer

it is much slower because it takes too long to calculate all the bishop's moves.

This position is only blocked from one side but the fact that it's a draw with best play is found immediately:

 FEN Viewer

3. Fortresses are definitely an area of chess where humans have an advantage over computers. However, this paper suggests that fortresses could be detected by the simple method of monitoring the evaluation function - if it doesn't change with increasing search depth, that indicates that the position is a fortress. This seems much simpler than using Monte-Carlo methods.

4. Thanks for the link, interesting.

5. Peter Svidler made a similar point in the commentary - not so much as proof that it was a draw but explaining why he could forecast a draw when the computer was saying -2. I think it's a useful idea but has limitations - it's not necessarily true, and there may be tries for the attacker where trying to maintain the fortress loses but abandoning the fortress saves the game.

6. Originally Posted by Ian Rout
... I think it's a useful idea but has limitations - it's not necessarily true, and there may be tries for the attacker where trying to maintain the fortress loses but abandoning the fortress saves the game.
Do you have examples?

7. I recently upgraded from Stockfish 8 to Stockfish 12 and noticed that the endgame evaluations were much better, e.g. an opposite-coloured bishops fortress being evaluated as 0.00 instead of +0.6 to the side with the extra pawns. Stockfish 8 after two minutes assesses the second position in Kevin's OP as +2.24, but Stockfish 12 gets to 0.00 in less than a second.

(I've never tried studying the Stockfish code, but since it's open source, it's in principle possible to work out what algorithm they're using for this.)