diff --git a/board/src/builder.rs b/board/src/builder.rs
index f52d18c..b20cd41 100644
--- a/board/src/builder.rs
+++ b/board/src/builder.rs
@@ -1,14 +1,13 @@
 // Eryn Wells <eryn@erynwells.me>
 
-use crate::{Board, Castle, EnPassant, Flags, PieceBitBoards};
-use chessfriend_core::{piece, Color, Piece, PlacedPiece, Rank, Shape, Square};
-use std::collections::BTreeMap;
+use crate::{piece_sets::Mailbox, Board, Castle, EnPassant, Flags, PieceSet};
+use chessfriend_core::{piece, Color, PlacedPiece, Rank, Shape, Square};
 
 #[derive(Clone)]
 pub struct Builder {
     player_to_move: Color,
     flags: Flags,
-    pieces: BTreeMap<Square, Piece>,
+    pieces: Mailbox,
     kings: [Option<Square>; 2],
     en_passant: Option<EnPassant>,
     ply_counter: u16,
@@ -23,11 +22,7 @@ impl Builder {
 
     #[must_use]
     pub fn from_board(board: &Board) -> Self {
-        let pieces = board
-            .pieces(Color::White)
-            .chain(board.pieces(Color::Black))
-            .map(|placed_piece| (placed_piece.square(), *placed_piece.piece()))
-            .collect::<BTreeMap<_, _>>();
+        let pieces = board.iter_all_pieces().collect::<Mailbox>();
 
         let white_king = board.king_square(Color::White);
         let black_king = board.king_square(Color::Black);
@@ -72,12 +67,12 @@ impl Builder {
             let color_index: usize = color as usize;
 
             if let Some(king_square) = self.kings[color_index] {
-                self.pieces.remove(&king_square);
+                self.pieces.remove(king_square);
             }
             self.kings[color_index] = Some(square);
         }
 
-        self.pieces.insert(square, *piece.piece());
+        self.pieces.set(piece.piece(), square);
 
         self
     }
@@ -108,7 +103,7 @@ impl Builder {
                 let parameters = castle.parameters(color);
                 let has_rook_on_starting_square = self
                     .pieces
-                    .get(&parameters.rook_origin_square())
+                    .get(parameters.rook_origin_square())
                     .is_some_and(|piece| piece.shape() == Shape::Rook);
                 let king_is_on_starting_square =
                     self.kings[color as usize] == Some(parameters.king_origin_square());
@@ -148,7 +143,7 @@ impl Default for Builder {
         let white_king_square = Square::E1;
         let black_king_square = Square::E8;
 
-        let pieces = BTreeMap::from_iter([
+        let pieces = Mailbox::from_iter([
             (white_king_square, piece!(White King)),
             (black_king_square, piece!(Black King)),
         ]);
diff --git a/board/src/piece_sets/mailbox.rs b/board/src/piece_sets/mailbox.rs
index c551fb6..05cdf9b 100644
--- a/board/src/piece_sets/mailbox.rs
+++ b/board/src/piece_sets/mailbox.rs
@@ -4,24 +4,34 @@ use chessfriend_core::{Piece, PlacedPiece, Square};
 use std::iter::FromIterator;
 
 #[derive(Clone, Debug, Eq, Hash, PartialEq)]
-pub(super) struct Mailbox([Option<Piece>; Square::NUM]);
+pub(crate) struct Mailbox([Option<Piece>; Square::NUM]);
 
 impl Mailbox {
-    pub(super) fn new() -> Self {
+    pub(crate) fn new() -> Self {
         Self::default()
     }
 
-    pub(super) fn get(&self, square: Square) -> Option<Piece> {
+    pub(crate) fn get(&self, square: Square) -> Option<Piece> {
         self.0[square as usize]
     }
 
-    pub(super) fn set(&mut self, square: Square, piece: Piece) {
+    pub(crate) fn set(&mut self, piece: Piece, square: Square) {
         self.0[square as usize] = Some(piece);
     }
 
-    pub(super) fn clear(&mut self, square: Square) {
+    pub(crate) fn remove(&mut self, square: Square) {
         self.0[square as usize] = None;
     }
+
+    pub(crate) fn iter(&self) -> impl Iterator<Item = PlacedPiece> {
+        self.0
+            .into_iter()
+            .flatten() // Remove the Nones
+            .zip(0u8..) // Enumerate with u8 instead of usize
+            .map(|(piece, index)| {
+                PlacedPiece::new(piece, unsafe { Square::from_index_unchecked(index) })
+            })
+    }
 }
 
 impl Default for Mailbox {
@@ -40,7 +50,7 @@ impl FromIterator<PlacedPiece> for Mailbox {
     fn from_iter<T: IntoIterator<Item = PlacedPiece>>(iter: T) -> Self {
         let mut mailbox = Self::new();
         for placed_piece in iter {
-            mailbox.set(placed_piece.square(), placed_piece.piece());
+            mailbox.set(placed_piece.piece(), placed_piece.square());
         }
 
         mailbox
@@ -51,7 +61,7 @@ impl FromIterator<(Square, Piece)> for Mailbox {
     fn from_iter<T: IntoIterator<Item = (Square, Piece)>>(iter: T) -> Self {
         let mut mailbox = Self::new();
         for (square, piece) in iter {
-            mailbox.set(square, piece);
+            mailbox.set(piece, square);
         }
 
         mailbox