diff --git a/Sources/Engine/Board.swift b/Sources/Engine/Board.swift
index 0203e54..88cd9e8 100644
--- a/Sources/Engine/Board.swift
+++ b/Sources/Engine/Board.swift
@@ -1,5 +1,5 @@
 enum Event {
-	case kingInCheck(_ by: Piece)
+	case kingInCheck(_ by: Piece, on: King)
 	case piecePinned(from: Piece, on: Piece)
 }
 protocol EventDelegate {
@@ -24,12 +24,30 @@ public class Board: CustomStringConvertible, EventDelegate {
 
 	}
 
+	public private(set) var halfMoveClock: UInt8 = 0
+
+	public var fullMoveClock: UInt8 {
+		halfMoveClock / 2
+	}
+
+	internal var threatenedSquares: [Color: Set<Square.Position>] = [
+		.White: [],
+		.Black: [],
+	]
+
+	public private(set) var turn: Color = .White
+
 	func movePiece(_ piece: Piece, to dst: Square.Position) throws {
 		let from = piece.position
 		piece.position = dst
 		squares[dst.index!].piece = piece
 		squares[from.index!].piece = nil
-		fen.set(from: board, castling: .All, enPassant: fen.enPassant)
+		halfMoveClock += 1
+		turn = !turn
+		fen.set(
+			from: board, activeColor: turn, castling: .All,
+			enPassant: fen.enPassant,
+			halfMoveClock: halfMoveClock, fullMoveClock: fullMoveClock)
 	}
 
 	public enum MoveFailure: Error, CustomStringConvertible {
@@ -105,6 +123,12 @@ public class Board: CustomStringConvertible, EventDelegate {
 		}
 
 		try piece.move(to: dst)
+		#warning("May need to clear threatenedSquares before hand.")
+		for square in squares where square.piece != nil {
+			let p = square.piece!
+			p.getLegalPosition()
+			threatenedSquares[p.color]! += Set(p.legalPositions)
+		}
 	}
 
 	public func getSquareInfo(on pos: Square.Position) -> Square? {
@@ -183,9 +207,6 @@ public class Board: CustomStringConvertible, EventDelegate {
 			file += 1
 			index += 1
 		}
-		#if DEBUG
-			print(b)
-		#endif
 		squares = b
 	}
 
@@ -206,7 +227,7 @@ public class Board: CustomStringConvertible, EventDelegate {
 				rank -= 1
 			}
 		}
-		// TODO: Handle better
+		#warning("Handle better.")
 		do {
 			try setBoard()
 		} catch Fen.FenError.NotAppropriateLength(let n, let column) {
diff --git a/Sources/Engine/Extensions/Set+Union.swift b/Sources/Engine/Extensions/Set+Union.swift
new file mode 100644
index 0000000..b28e31c
--- /dev/null
+++ b/Sources/Engine/Extensions/Set+Union.swift
@@ -0,0 +1,9 @@
+extension Set {
+	static func + (lhs: Set<Element>, rhs: Set<Element>) -> Set<Element> {
+		return lhs.union(rhs)
+	}
+
+	static func += (lhs: inout Set<Element>, rhs: Set<Element>) {
+		lhs.formUnion(rhs)
+	}
+}
diff --git a/Sources/Engine/FEN.swift b/Sources/Engine/FEN.swift
index a836749..c84a532 100644
--- a/Sources/Engine/FEN.swift
+++ b/Sources/Engine/FEN.swift
@@ -64,8 +64,9 @@ public struct Fen: CustomStringConvertible {
 	}
 
 	internal mutating func set(
-		from board: Board.Grid, castling ca: CastlingAvailibility,
-		enPassant: String
+		from board: Board.Grid, activeColor: Color,
+		castling ca: CastlingAvailibility,
+		enPassant: String, halfMoveClock hmc: UInt8, fullMoveClock fmc: UInt8
 	) {
 		#warning(
 			"Determine active color, halfMoveClock, fullMoveClock based on history SAN later passed in arguments"
@@ -95,7 +96,8 @@ public struct Fen: CustomStringConvertible {
 			rankNr += 1
 		}
 
-		value = placement + " w " + ca.rawValue + " " + enPassant + " 0 " + "1"
+		value =
+			"\(placement) \(activeColor) \(ca.rawValue) \(enPassant) \(halfMoveClock) \(fullMoveClock)"
 	}
 
 	public enum FenError: Error {
diff --git a/Sources/Engine/Pieces/Bishop.swift b/Sources/Engine/Pieces/Bishop.swift
index 74c5840..af1047a 100644
--- a/Sources/Engine/Pieces/Bishop.swift
+++ b/Sources/Engine/Pieces/Bishop.swift
@@ -8,10 +8,6 @@ final class Bishop: Piece, DiagonalMoves {
 		return getDiagonalMoves(from: position)
 	}
 
-	override var legalPositions: [Square.Position] {
-		return pseudoLegalPositions.filter { isLegal(on: $0) }
-	}
-
 	override func isLegal(on pos: Square.Position) -> Bool {
 		super.isLegal(on: pos)
 	}
diff --git a/Sources/Engine/Pieces/King.swift b/Sources/Engine/Pieces/King.swift
index ebc4091..b95af31 100644
--- a/Sources/Engine/Pieces/King.swift
+++ b/Sources/Engine/Pieces/King.swift
@@ -1,5 +1,6 @@
 final class King: Piece {
-
+	typealias Threats = (Piece?, Piece?)
+	var threats: Threats = (nil, nil)
 	override var unicodeRepresentation: String {
 		return color == .Black ? "♛" : "♕"
 	}
@@ -17,12 +18,12 @@ final class King: Piece {
 		].filter { $0.index != nil }
 	}
 
-	override var legalPositions: [Square.Position] {
-		return pseudoLegalPositions.filter { isLegal(on: $0) }
-	}
-
 	override func isLegal(on pos: Square.Position) -> Bool {
-		super.isLegal(on: pos)
+		guard super.isLegal(on: pos) else {
+			return false
+		}
+		// Do stuff
+		return true
 	}
 
 	init(with color: Color, on position: Square.Position) {
diff --git a/Sources/Engine/Pieces/Knight.swift b/Sources/Engine/Pieces/Knight.swift
index 2c14116..6682d67 100644
--- a/Sources/Engine/Pieces/Knight.swift
+++ b/Sources/Engine/Pieces/Knight.swift
@@ -16,10 +16,6 @@ final class Knight: Piece {
 		].filter { $0.index != nil }
 	}
 
-	override var legalPositions: [Square.Position] {
-		return pseudoLegalPositions.filter { isLegal(on: $0) }
-	}
-
 	override func isLegal(on pos: Square.Position) -> Bool {
 		super.isLegal(on: pos)
 	}
diff --git a/Sources/Engine/Pieces/Pawn.swift b/Sources/Engine/Pieces/Pawn.swift
index 517331c..fa2d50b 100644
--- a/Sources/Engine/Pieces/Pawn.swift
+++ b/Sources/Engine/Pieces/Pawn.swift
@@ -9,10 +9,6 @@ final class Pawn: Piece {
 		].filter { $0.index != nil }
 	}
 
-	override var legalPositions: [Square.Position] {
-		pseudoLegalPositions.filter { isLegal(on: $0) }
-	}
-
 	override var unicodeRepresentation: String {
 		color == .Black ? "♟" : "♙"
 	}
diff --git a/Sources/Engine/Pieces/Piece.swift b/Sources/Engine/Pieces/Piece.swift
index 4c04b78..5023495 100644
--- a/Sources/Engine/Pieces/Piece.swift
+++ b/Sources/Engine/Pieces/Piece.swift
@@ -54,9 +54,10 @@ public enum Kind: String, CaseIterable {
 }
 
 public class Piece {
-	#warning("TODO: TO be removed, handle everything through the delegate")
+	#warning("TODO: To be removed, handle everything through the delegate")
 	internal weak var board: Board?
 	public internal(set) var color: Color
+	public internal(set) var halfMoveCount: UInt8 = 0
 	public var unicodeRepresentation: String {
 		return ""
 	}
@@ -65,25 +66,29 @@ public class Piece {
 	internal var pseudoLegalPositions: [Square.Position] {
 		return []
 	}
-	internal var legalPositions: [Square.Position] {
-		return []
+	internal var legalPositions = [Square.Position]()
+	internal func getLegalPosition() {
+		legalPositions = pseudoLegalPositions.filter { isLegal(on: $0) }
 	}
 	internal var delegate: EventDelegate?
+
 	internal func move(to dst: Square.Position) throws {
 		if !(legalPositions.contains { $0 == dst }) {
 			throw Board.MoveFailure.destinationIsIllegal(pos: dst)
 		}
 
 		try delegate?.movePiece(self, to: dst)
+		halfMoveCount += 1
 	}
-	internal func isLegal(on pos: Square.Position) -> Bool {
 
+	#warning("This method should be better thought out.")
+	internal func isLegal(on pos: Square.Position) -> Bool {
 		if let board = board, let s = board[pos] {
 			if let p = s.piece {
 				if p.color == color { return false }
-				if p.kind == .King {
-					// TODO: Notify board of check
-					delegate?.notify(.kingInCheck(self))
+				if let king = p as? King {
+					#warning("It could be in check or behind another piece.")
+					delegate?.notify(.kingInCheck(self, on: king))
 					return false
 				}
 			}
diff --git a/Sources/Engine/Pieces/Queen.swift b/Sources/Engine/Pieces/Queen.swift
index 6a1a54d..b5e375a 100644
--- a/Sources/Engine/Pieces/Queen.swift
+++ b/Sources/Engine/Pieces/Queen.swift
@@ -7,10 +7,6 @@ final class Queen: Piece, LinearMoves, DiagonalMoves {
 		return getDiagonalMoves(from: position) + getLinearMoves(from: position)
 	}
 
-	override var legalPositions: [Square.Position] {
-		return pseudoLegalPositions.filter { isLegal(on: $0) }
-	}
-
 	override func isLegal(on pos: Square.Position) -> Bool {
 		super.isLegal(on: pos)
 	}
diff --git a/Sources/Engine/Pieces/Rook.swift b/Sources/Engine/Pieces/Rook.swift
index 6c9f2f1..0ed9fba 100644
--- a/Sources/Engine/Pieces/Rook.swift
+++ b/Sources/Engine/Pieces/Rook.swift
@@ -8,10 +8,6 @@ final class Rook: Piece, LinearMoves {
 		return getLinearMoves(from: position)
 	}
 
-	override var legalPositions: [Square.Position] {
-		return pseudoLegalPositions.filter { isLegal(on: $0) }
-	}
-
 	override func isLegal(on pos: Square.Position) -> Bool {
 		super.isLegal(on: pos)
 	}
diff --git a/Sources/Engine/Square.swift b/Sources/Engine/Square.swift
index 79aaa91..22165c5 100644
--- a/Sources/Engine/Square.swift
+++ b/Sources/Engine/Square.swift
@@ -1,6 +1,6 @@
 public struct Square: Equatable {
 
-	public struct Position: Equatable {
+	public struct Position: Equatable, Hashable {
 		public let rank: Int8
 		public let file: Int8
 
diff --git a/Sources/Engine/utils.swift b/Sources/Engine/utils.swift
index d15222a..02ccaad 100644
--- a/Sources/Engine/utils.swift
+++ b/Sources/Engine/utils.swift
@@ -1,4 +1,15 @@
-public enum Color: UInt8 {
+public enum Color: UInt8, CustomStringConvertible {
 	case Black = 0
 	case White = 1
+
+	public var description: String {
+		return switch self {
+		case .Black: "b"
+		case .White: "w"
+		}
+	}
+
+	public static prefix func ! (rhs: Self) -> Self {
+		return rhs == .White ? .Black : .White
+	}
 }
diff --git a/Tests/EngineTests/EngineTests.swift b/Tests/EngineTests/EngineTests.swift
index 4018a90..4ab67a6 100644
--- a/Tests/EngineTests/EngineTests.swift
+++ b/Tests/EngineTests/EngineTests.swift
@@ -109,8 +109,11 @@ final class EngineTests: XCTestCase {
 		let board: Board = .init()
 		let fen = board.fen.value
 		board.fen.set(
-			from: board.board, castling: board.fen.castlingAvailibility,
-			enPassant: board.fen.enPassant)
+			from: board.board, activeColor: board.fen.activeColor,
+			castling: board.fen.castlingAvailibility,
+			enPassant: board.fen.enPassant,
+			halfMoveClock:
+				board.fen.halfMoveClock, fullMoveClock: board.fen.fullMoveClock)
 		XCTAssertEqual(
 			fen, board.fen.value, "Expected \(fen) got \(board.fen.value)")
 	}