diff --git a/solitaire_core/src/deck.rs b/solitaire_core/src/deck.rs
new file mode 100644
index 0000000..8d2e391
--- /dev/null
+++ b/solitaire_core/src/deck.rs
@@ -0,0 +1,159 @@
+use rand::{seq::SliceRandom, SeedableRng};
+use rand::rngs::StdRng;
+use crate::card::{Card, Rank, Suit};
+use crate::pile::{Pile, PileType};
+
+const ALL_SUITS: [Suit; 4] = [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades];
+const ALL_RANKS: [Rank; 13] = [
+    Rank::Ace, Rank::Two, Rank::Three, Rank::Four, Rank::Five,
+    Rank::Six, Rank::Seven, Rank::Eight, Rank::Nine, Rank::Ten,
+    Rank::Jack, Rank::Queen, Rank::King,
+];
+
+/// A standard 52-card deck.
+pub struct Deck {
+    pub cards: Vec<Card>,
+}
+
+impl Deck {
+    /// Creates an unshuffled deck with all 52 unique cards (id 0–51).
+    pub fn new() -> Self {
+        let mut cards = Vec::with_capacity(52);
+        let mut id = 0u32;
+        for &suit in &ALL_SUITS {
+            for &rank in &ALL_RANKS {
+                cards.push(Card { id, suit, rank, face_up: false });
+                id += 1;
+            }
+        }
+        Self { cards }
+    }
+
+    /// Shuffles the deck in-place using Fisher-Yates with a seeded `SmallRng`.
+    /// The same seed always produces the same order on any platform.
+    pub fn shuffle(&mut self, seed: u64) {
+        let mut rng = StdRng::seed_from_u64(seed);
+        self.cards.shuffle(&mut rng);
+    }
+}
+
+impl Default for Deck {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+/// Deals a standard Klondike layout from a pre-shuffled deck.
+///
+/// Returns 7 tableau piles and the remaining stock pile.
+/// Column `i` contains `i + 1` cards; only the top card is face-up.
+/// Stock receives the remaining 24 cards, all face-down.
+pub fn deal_klondike(deck: Deck) -> ([Pile; 7], Pile) {
+    let mut tableau: [Pile; 7] = core::array::from_fn(|i| Pile::new(PileType::Tableau(i)));
+    let mut cards = deck.cards.into_iter();
+
+    for (col, pile) in tableau.iter_mut().enumerate() {
+        for row in 0..=col {
+            let mut card = cards.next().expect("deck has 52 cards");
+            card.face_up = row == col;
+            pile.cards.push(card);
+        }
+    }
+
+    let mut stock = Pile::new(PileType::Stock);
+    stock.cards.extend(cards);
+    (tableau, stock)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn deck_new_has_52_cards() {
+        assert_eq!(Deck::new().cards.len(), 52);
+    }
+
+    #[test]
+    fn deck_new_has_unique_ids() {
+        let deck = Deck::new();
+        let mut ids: Vec<u32> = deck.cards.iter().map(|c| c.id).collect();
+        ids.sort_unstable();
+        ids.dedup();
+        assert_eq!(ids.len(), 52);
+    }
+
+    #[test]
+    fn deck_new_has_all_suits_and_ranks() {
+        let deck = Deck::new();
+        for suit in ALL_SUITS {
+            for rank in ALL_RANKS {
+                assert!(
+                    deck.cards.iter().any(|c| c.suit == suit && c.rank == rank),
+                    "missing {rank:?} {suit:?}"
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn same_seed_produces_same_order() {
+        let mut d1 = Deck::new(); d1.shuffle(42);
+        let mut d2 = Deck::new(); d2.shuffle(42);
+        assert_eq!(d1.cards, d2.cards);
+    }
+
+    #[test]
+    fn different_seeds_produce_different_orders() {
+        let mut d1 = Deck::new(); d1.shuffle(1);
+        let mut d2 = Deck::new(); d2.shuffle(2);
+        assert_ne!(d1.cards, d2.cards);
+    }
+
+    #[test]
+    fn deal_klondike_correct_tableau_sizes() {
+        let mut deck = Deck::new(); deck.shuffle(0);
+        let (tableau, stock) = deal_klondike(deck);
+        for (i, pile) in tableau.iter().enumerate() {
+            assert_eq!(pile.cards.len(), i + 1, "col {i} wrong size");
+        }
+        assert_eq!(stock.cards.len(), 24);
+    }
+
+    #[test]
+    fn deal_klondike_top_cards_are_face_up() {
+        let mut deck = Deck::new(); deck.shuffle(0);
+        let (tableau, _) = deal_klondike(deck);
+        for pile in &tableau {
+            assert!(pile.cards.last().unwrap().face_up);
+        }
+    }
+
+    #[test]
+    fn deal_klondike_non_top_cards_are_face_down() {
+        let mut deck = Deck::new(); deck.shuffle(0);
+        let (tableau, _) = deal_klondike(deck);
+        for pile in &tableau {
+            for card in &pile.cards[..pile.cards.len().saturating_sub(1)] {
+                assert!(!card.face_up);
+            }
+        }
+    }
+
+    #[test]
+    fn deal_klondike_stock_is_face_down() {
+        let mut deck = Deck::new(); deck.shuffle(0);
+        let (_, stock) = deal_klondike(deck);
+        assert!(stock.cards.iter().all(|c| !c.face_up));
+    }
+
+    #[test]
+    fn deal_klondike_all_52_cards_present() {
+        let mut deck = Deck::new(); deck.shuffle(99);
+        let (tableau, stock) = deal_klondike(deck);
+        let mut ids: Vec<u32> = stock.cards.iter().map(|c| c.id).collect();
+        for pile in &tableau { ids.extend(pile.cards.iter().map(|c| c.id)); }
+        ids.sort_unstable();
+        assert_eq!(ids, (0u32..52).collect::<Vec<_>>());
+    }
+}
diff --git a/solitaire_core/src/error.rs b/solitaire_core/src/error.rs
new file mode 100644
index 0000000..6566a87
--- /dev/null
+++ b/solitaire_core/src/error.rs
@@ -0,0 +1,36 @@
+use thiserror::Error;
+
+/// All reasons a game move can be rejected.
+#[derive(Debug, Clone, PartialEq, Eq, Error)]
+pub enum MoveError {
+    #[error("invalid source pile")]
+    InvalidSource,
+    #[error("invalid destination pile")]
+    InvalidDestination,
+    #[error("source pile is empty")]
+    EmptySource,
+    #[error("move violates rules: {0}")]
+    RuleViolation(String),
+    #[error("undo stack is empty")]
+    UndoStackEmpty,
+    #[error("game is already won")]
+    GameAlreadyWon,
+    #[error("stock and waste are both empty")]
+    StockEmpty,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rule_violation_includes_message() {
+        let e = MoveError::RuleViolation("king only on empty".into());
+        assert!(e.to_string().contains("king only on empty"));
+    }
+
+    #[test]
+    fn undo_stack_empty_has_non_empty_message() {
+        assert!(!MoveError::UndoStackEmpty.to_string().is_empty());
+    }
+}
diff --git a/solitaire_core/src/lib.rs b/solitaire_core/src/lib.rs
index b4ba26a..79d71b0 100644
--- a/solitaire_core/src/lib.rs
+++ b/solitaire_core/src/lib.rs
@@ -1 +1,6 @@
 pub mod card;
+pub mod deck;
+pub mod error;
+pub mod pile;
+pub mod rules;
+pub mod scoring;
diff --git a/solitaire_core/src/pile.rs b/solitaire_core/src/pile.rs
new file mode 100644
index 0000000..de3c51f
--- /dev/null
+++ b/solitaire_core/src/pile.rs
@@ -0,0 +1,70 @@
+use serde::{Deserialize, Serialize};
+use crate::card::{Card, Suit};
+
+/// Identifies which pile on the board a set of cards belongs to.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum PileType {
+    /// The face-down draw pile.
+    Stock,
+    /// The face-up discard pile drawn to.
+    Waste,
+    /// One of the four suit-ordered foundation piles.
+    Foundation(Suit),
+    /// One of the seven tableau columns (0–6).
+    Tableau(usize),
+}
+
+/// A named collection of cards in a specific board position.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct Pile {
+    pub pile_type: PileType,
+    pub cards: Vec<Card>,
+}
+
+impl Pile {
+    /// Creates a new empty pile of the given type.
+    pub fn new(pile_type: PileType) -> Self {
+        Self { pile_type, cards: Vec::new() }
+    }
+
+    /// Returns a reference to the top (last) card, or `None` if empty.
+    pub fn top(&self) -> Option<&Card> {
+        self.cards.last()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::card::{Card, Rank, Suit};
+
+    #[test]
+    fn new_pile_is_empty() {
+        let pile = Pile::new(PileType::Stock);
+        assert!(pile.cards.is_empty());
+    }
+
+    #[test]
+    fn pile_top_returns_last_card() {
+        let mut pile = Pile::new(PileType::Waste);
+        pile.cards.push(Card { id: 0, suit: Suit::Hearts, rank: Rank::Ace, face_up: true });
+        pile.cards.push(Card { id: 1, suit: Suit::Clubs, rank: Rank::Two, face_up: true });
+        assert_eq!(pile.top().unwrap().id, 1);
+    }
+
+    #[test]
+    fn pile_top_on_empty_is_none() {
+        let pile = Pile::new(PileType::Waste);
+        assert!(pile.top().is_none());
+    }
+
+    #[test]
+    fn pile_type_foundation_uses_suit() {
+        assert_ne!(PileType::Foundation(Suit::Hearts), PileType::Foundation(Suit::Spades));
+    }
+
+    #[test]
+    fn pile_type_tableau_uses_index() {
+        assert_ne!(PileType::Tableau(0), PileType::Tableau(6));
+    }
+}
diff --git a/solitaire_core/src/rules.rs b/solitaire_core/src/rules.rs
new file mode 100644
index 0000000..b82a963
--- /dev/null
+++ b/solitaire_core/src/rules.rs
@@ -0,0 +1,122 @@
+use crate::card::{Card, Suit};
+use crate::pile::Pile;
+
+/// Returns `true` if `card` can be placed on `pile` as the next card in the foundation for `suit`.
+///
+/// Foundation rules: same suit, Ace starts, each subsequent card is one rank higher.
+pub fn can_place_on_foundation(card: &Card, pile: &Pile, suit: Suit) -> bool {
+    if card.suit != suit {
+        return false;
+    }
+    match pile.cards.last() {
+        None => card.rank.value() == 1,
+        Some(top) => card.rank.value() == top.rank.value() + 1,
+    }
+}
+
+/// Returns `true` if `card` (or the bottom card of a sequence) can be placed on `pile` in the tableau.
+///
+/// Tableau rules: Kings go on empty piles; otherwise alternating colour, one rank lower.
+pub fn can_place_on_tableau(card: &Card, pile: &Pile) -> bool {
+    match pile.cards.last() {
+        None => card.rank.value() == 13,
+        Some(top) => {
+            card.rank.value() + 1 == top.rank.value()
+                && card.suit.is_red() != top.suit.is_red()
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::card::{Card, Rank, Suit};
+    use crate::pile::{Pile, PileType};
+
+    fn card(suit: Suit, rank: Rank) -> Card {
+        Card { id: 0, suit, rank, face_up: true }
+    }
+
+    fn pile_with(pile_type: PileType, cards: Vec<Card>) -> Pile {
+        Pile { pile_type, cards }
+    }
+
+    // Foundation tests
+    #[test]
+    fn foundation_ace_on_empty_is_valid() {
+        let c = card(Suit::Hearts, Rank::Ace);
+        let p = Pile::new(PileType::Foundation(Suit::Hearts));
+        assert!(can_place_on_foundation(&c, &p, Suit::Hearts));
+    }
+
+    #[test]
+    fn foundation_non_ace_on_empty_is_invalid() {
+        let c = card(Suit::Hearts, Rank::Two);
+        let p = Pile::new(PileType::Foundation(Suit::Hearts));
+        assert!(!can_place_on_foundation(&c, &p, Suit::Hearts));
+    }
+
+    #[test]
+    fn foundation_two_on_ace_same_suit_is_valid() {
+        let c = card(Suit::Clubs, Rank::Two);
+        let p = pile_with(PileType::Foundation(Suit::Clubs), vec![card(Suit::Clubs, Rank::Ace)]);
+        assert!(can_place_on_foundation(&c, &p, Suit::Clubs));
+    }
+
+    #[test]
+    fn foundation_wrong_suit_is_invalid() {
+        let c = card(Suit::Hearts, Rank::Ace);
+        let p = Pile::new(PileType::Foundation(Suit::Spades));
+        assert!(!can_place_on_foundation(&c, &p, Suit::Spades));
+    }
+
+    #[test]
+    fn foundation_skipping_rank_is_invalid() {
+        let c = card(Suit::Diamonds, Rank::Three);
+        let p = pile_with(PileType::Foundation(Suit::Diamonds), vec![card(Suit::Diamonds, Rank::Ace)]);
+        assert!(!can_place_on_foundation(&c, &p, Suit::Diamonds));
+    }
+
+    // Tableau tests
+    #[test]
+    fn tableau_king_on_empty_is_valid() {
+        let c = card(Suit::Hearts, Rank::King);
+        let p = Pile::new(PileType::Tableau(0));
+        assert!(can_place_on_tableau(&c, &p));
+    }
+
+    #[test]
+    fn tableau_non_king_on_empty_is_invalid() {
+        let c = card(Suit::Hearts, Rank::Queen);
+        let p = Pile::new(PileType::Tableau(0));
+        assert!(!can_place_on_tableau(&c, &p));
+    }
+
+    #[test]
+    fn tableau_red_on_black_one_lower_is_valid() {
+        let c = card(Suit::Hearts, Rank::Nine);
+        let p = pile_with(PileType::Tableau(0), vec![card(Suit::Spades, Rank::Ten)]);
+        assert!(can_place_on_tableau(&c, &p));
+    }
+
+    #[test]
+    fn tableau_same_color_is_invalid() {
+        let c = card(Suit::Clubs, Rank::Nine);
+        let p = pile_with(PileType::Tableau(0), vec![card(Suit::Spades, Rank::Ten)]);
+        assert!(!can_place_on_tableau(&c, &p));
+    }
+
+    #[test]
+    fn tableau_wrong_rank_difference_is_invalid() {
+        let c = card(Suit::Hearts, Rank::Eight);
+        let p = pile_with(PileType::Tableau(0), vec![card(Suit::Spades, Rank::Ten)]);
+        assert!(!can_place_on_tableau(&c, &p));
+    }
+
+    #[test]
+    fn tableau_black_on_red_one_lower_is_valid() {
+        let c = card(Suit::Clubs, Rank::Six);
+        let p = pile_with(PileType::Tableau(0), vec![card(Suit::Hearts, Rank::Seven)]);
+        assert!(can_place_on_tableau(&c, &p));
+    }
+}
diff --git a/solitaire_core/src/scoring.rs b/solitaire_core/src/scoring.rs
new file mode 100644
index 0000000..2598f02
--- /dev/null
+++ b/solitaire_core/src/scoring.rs
@@ -0,0 +1,73 @@
+use crate::pile::PileType;
+
+/// Score delta for moving cards from `from` to `to`.
+///
+/// Windows XP Standard scoring:
+/// - +10 for any card reaching a foundation pile
+/// - +5 for a waste → tableau move
+/// - 0 for all other moves
+pub fn score_move(from: &PileType, to: &PileType) -> i32 {
+    match to {
+        PileType::Foundation(_) => 10,
+        PileType::Tableau(_) => {
+            if matches!(from, PileType::Waste) { 5 } else { 0 }
+        }
+        _ => 0,
+    }
+}
+
+/// Score penalty applied when the player uses undo: -15.
+pub fn score_undo() -> i32 {
+    -15
+}
+
+/// Time bonus added to the score on a win: `700_000 / elapsed_seconds`.
+/// Returns 0 when `elapsed_seconds` is 0 to avoid division by zero.
+pub fn compute_time_bonus(elapsed_seconds: u64) -> i32 {
+    if elapsed_seconds == 0 {
+        return 0;
+    }
+    (700_000u64 / elapsed_seconds).min(i32::MAX as u64) as i32
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::card::Suit;
+
+    #[test]
+    fn move_to_foundation_scores_ten() {
+        assert_eq!(score_move(&PileType::Waste, &PileType::Foundation(Suit::Hearts)), 10);
+        assert_eq!(score_move(&PileType::Tableau(0), &PileType::Foundation(Suit::Clubs)), 10);
+    }
+
+    #[test]
+    fn waste_to_tableau_scores_five() {
+        assert_eq!(score_move(&PileType::Waste, &PileType::Tableau(3)), 5);
+    }
+
+    #[test]
+    fn tableau_to_tableau_scores_zero() {
+        assert_eq!(score_move(&PileType::Tableau(0), &PileType::Tableau(1)), 0);
+    }
+
+    #[test]
+    fn undo_penalty_is_negative_fifteen() {
+        assert_eq!(score_undo(), -15);
+    }
+
+    #[test]
+    fn time_bonus_at_100_seconds() {
+        assert_eq!(compute_time_bonus(100), 7000);
+    }
+
+    #[test]
+    fn time_bonus_at_zero_is_zero() {
+        assert_eq!(compute_time_bonus(0), 0);
+    }
+
+    #[test]
+    fn time_bonus_at_one_second() {
+        assert_eq!(compute_time_bonus(1), 700_000);
+    }
+}