The black king has fallen off the board! Put him back in the correct place so that white is one move from checkmate and it is a position that could have been legally reached.

There are two legal answers.

Does white still have kingside castling available?

 

King f4

King_William wrote: King f4

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 If you mean 1.Rf1+, then 1... Bf3.

 

May be King at c1? 

Can't be king on c1... if the white king has moved then no checkmate because he cannot castle...if the white king has not moved then how could the black king get there legally?

Kingskiller wrote: King_William wrote: King f4

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 If you mean 1.Rf1+, then 1... Bf3.

 

May be King at c1? 

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 Yeah f4 is not an option.

I think black king on f3 is one solution... this gives 0-0#.

Oh yes, there is only one solution, sorry. I forgot f4 could have the attack blocked.

king c1?

f3

I like the concept this puzzle is based on.. I had to read the comments to figure it out, though.

f4- white mates by either 0-0# (if possible) or Rhf1#

c1- "   " by Ke2#

 

Locke wrote: f3c1 is not a viable answer as that position cannot have been reached if the white king has not moved, and if he has mate is not achievable.

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Yeah, c1 would be a possible solution, but I cannot see how the position could be reached.  f3 seems the only viable solution assuming a castle is possible. without the ability to castle, black can defend ... for a few moves anyway.

Frankly, I don't believe the black king "fell" off the board, I think black slapped it off in a dramatic resignation move...