Fortress plus two lifesaving cells: a single-deck escape act with medium difficulty and an inviting 80% win rate.

Bastion Solitaire takes Fortress Solitaire and bolts on two storage cells, just enough breathing room to escape tight spots. It's a single-deck game of medium difficulty, an 80% chance of winning, and your odds come down mostly to skill.
Other members of the fortress family are worth a look too: Beleaguered Castle Solitaire, Streets and Alleys Solitaire, Citadel, Chessboard, Ten By One, and Fortress Moat.
Give it a shot if a two-cell safety net sounds like your kind of challenge.
If you run into anything odd or have an idea that would make the game better, please contact me.
Enjoy yourself!
4 foundation piles: Build up from Ace to King in the same suit.
10 tableau piles: Ten piles that build up or down by suit. Only one card moves at a time. Any available card can fill an open space. Each pile starts with 5 cards face-up.
2 cells: Two storage cells, one card apiece to start. Each cell holds only one card at a time.
There are four foundation piles.
Any ace can be placed on an empty foundation pile.
A card joins a foundation pile only if it's one rank higher than the pile's top card and the same suit, so the only card that can land on a six of hearts is the seven of hearts.
The top card of any foundation pile can be moved back into play if another pile will take it.
Two cells, each holding one card at the start.
Any available card can move into an open cell. A card sitting in a cell can move back to the tableau or up to the foundation.
Ten tableau piles hold five cards each, all dealt face-up.
A card joins a tableau pile only if it's one rank above or below the pile's top card and the same suit, so the cards that fit on a seven of spades are the six or eight of spades.
Any uncovered tableau card can move to the foundation, an empty cell, or another tableau pile.
Any card can fill an empty tableau slot.
Technically only one card moves at a time, but since super moves are allowed, you can shuffle a whole sequence across in one go whenever there are enough empty cells or tableau columns to cover it.