diff --git a/gauguin-core/src/test/kotlin/org/piepmeyer/gauguin/difficulty/human/HumanDifficultySolverTest.kt b/gauguin-core/src/test/kotlin/org/piepmeyer/gauguin/difficulty/human/HumanDifficultySolverTest.kt
index 4c35765a..3d8c0c49 100644
--- a/gauguin-core/src/test/kotlin/org/piepmeyer/gauguin/difficulty/human/HumanDifficultySolverTest.kt
+++ b/gauguin-core/src/test/kotlin/org/piepmeyer/gauguin/difficulty/human/HumanDifficultySolverTest.kt
@@ -19,6 +19,7 @@ class HumanDifficultySolverTest :
             // 10_000 of 5x5, merge: 134 left unsolved
             // 10_000 of 2x4, merge: no (!) left unsolved
             // 1_000 of 3x6, merge: 125 left unsolved
+            //  100 of 9x9, merge: 51 left unsolved
             withClue("seed $seed") {
                 test("seed random grid should be solved") {
                     val randomizer = SeedRandomizerMock(seed)
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_2-doppelte-zwei-nicht-moeglich-.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-doppelte-zwei-nicht-moeglich.yml
similarity index 100%
rename from gauguin-core/src/test/resources/difficulty-balancing/game_2-doppelte-zwei-nicht-moeglich-.yml
rename to gauguin-core/src/test/resources/difficulty-balancing/game_4x4-doppelte-zwei-nicht-moeglich.yml
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-and-smooth.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-and-smooth.yml
new file mode 100644
index 00000000..30a143d7
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-and-smooth.yml
@@ -0,0 +1 @@
+{"variant":{"gridSize":{"width":4,"height":4},"options":{"showOperators":true,"cageOperation":"OPERATIONS_ALL","digitSetting":"FIRST_DIGIT_ONE","difficultySetting":"EXTREME","singleCageUsage":"FIXED_NUMBER","numeralSystem":"Decimal"}},"savedAtInMilliseconds":1723319581539,"playTimeInMilliseconds":56239,"startedToBePlayed":true,"isActive":false,"cells":[{"cellNumber":0,"row":0,"column":0,"value":3,"userValue":3,"possibles":[]},{"cellNumber":1,"row":0,"column":1,"value":2,"userValue":2,"possibles":[]},{"cellNumber":2,"row":0,"column":2,"value":4,"userValue":4,"possibles":[]},{"cellNumber":3,"row":0,"column":3,"value":1,"userValue":1,"possibles":[]},{"cellNumber":4,"row":1,"column":0,"value":2,"userValue":2,"possibles":[]},{"cellNumber":5,"row":1,"column":1,"value":3,"userValue":3,"possibles":[]},{"cellNumber":6,"row":1,"column":2,"value":1,"userValue":1,"possibles":[]},{"cellNumber":7,"row":1,"column":3,"value":4,"userValue":4,"possibles":[]},{"cellNumber":8,"row":2,"column":0,"value":4,"userValue":4,"possibles":[]},{"cellNumber":9,"row":2,"column":1,"value":1,"userValue":1,"possibles":[]},{"cellNumber":10,"row":2,"column":2,"value":3,"userValue":3,"possibles":[]},{"cellNumber":11,"row":2,"column":3,"value":2,"userValue":2,"possibles":[]},{"cellNumber":12,"row":3,"column":0,"value":1,"userValue":1,"possibles":[]},{"cellNumber":13,"row":3,"column":1,"value":4,"userValue":4,"possibles":[]},{"cellNumber":14,"row":3,"column":2,"value":2,"userValue":2,"possibles":[]},{"cellNumber":15,"row":3,"column":3,"value":3,"userValue":3,"possibles":[]}],"selectedCellNumber":4,"invalidCellNumbers":[],"cheatedCellNumbers":[],"cages":[{"id":0,"action":"ACTION_SUBTRACT","type":"DOUBLE_HORIZONTAL","result":3,"cellNumbers":[12,13]},{"id":1,"action":"ACTION_ADD","type":"ANGLE_LEFT_TOP","result":7,"cellNumbers":[11,15,14]},{"id":2,"action":"ACTION_ADD","type":"ANGLE_LEFT_TOP","result":5,"cellNumbers":[6,10,9]},{"id":3,"action":"ACTION_MULTIPLY","type":"TETRIS_T_LEFT_UP","result":72,"cellNumbers":[0,4,5,8]},{"id":4,"action":"ACTION_MULTIPLY","type":"L_HORIZONTAL_SHORT_RIGHT_BOTTOM","result":32,"cellNumbers":[1,2,3,7]}]}
\ No newline at end of file
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-may-be-missing-xwing.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-may-be-missing-xwing.yml
new file mode 100644
index 00000000..0754379b
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-may-be-missing-xwing.yml
@@ -0,0 +1 @@
+{"variant":{"gridSize":{"width":4,"height":4},"options":{"showOperators":true,"cageOperation":"OPERATIONS_ALL","digitSetting":"FIRST_DIGIT_ONE","difficultySetting":"EXTREME","singleCageUsage":"FIXED_NUMBER","numeralSystem":"Decimal"}},"savedAtInMilliseconds":1723716757705,"playTimeInMilliseconds":47244,"startedToBePlayed":true,"isActive":false,"cells":[{"cellNumber":0,"row":0,"column":0,"value":4,"userValue":4,"possibles":[]},{"cellNumber":1,"row":0,"column":1,"value":2,"userValue":2,"possibles":[]},{"cellNumber":2,"row":0,"column":2,"value":1,"userValue":1,"possibles":[]},{"cellNumber":3,"row":0,"column":3,"value":3,"userValue":3,"possibles":[]},{"cellNumber":4,"row":1,"column":0,"value":1,"userValue":1,"possibles":[]},{"cellNumber":5,"row":1,"column":1,"value":4,"userValue":4,"possibles":[]},{"cellNumber":6,"row":1,"column":2,"value":3,"userValue":3,"possibles":[]},{"cellNumber":7,"row":1,"column":3,"value":2,"userValue":2,"possibles":[]},{"cellNumber":8,"row":2,"column":0,"value":3,"userValue":3,"possibles":[]},{"cellNumber":9,"row":2,"column":1,"value":1,"userValue":1,"possibles":[]},{"cellNumber":10,"row":2,"column":2,"value":2,"userValue":2,"possibles":[]},{"cellNumber":11,"row":2,"column":3,"value":4,"userValue":4,"possibles":[]},{"cellNumber":12,"row":3,"column":0,"value":2,"userValue":2,"possibles":[]},{"cellNumber":13,"row":3,"column":1,"value":3,"userValue":3,"possibles":[]},{"cellNumber":14,"row":3,"column":2,"value":4,"userValue":4,"possibles":[]},{"cellNumber":15,"row":3,"column":3,"value":1,"userValue":1,"possibles":[]}],"selectedCellNumber":14,"invalidCellNumbers":[],"cheatedCellNumbers":[],"cages":[{"id":0,"action":"ACTION_ADD","type":"DOUBLE_VERTICAL","result":5,"cellNumbers":[0,4]},{"id":1,"action":"ACTION_ADD","type":"ANGLE_LEFT_TOP","result":9,"cellNumbers":[11,15,14]},{"id":2,"action":"ACTION_MULTIPLY","type":"SQUARE","result":18,"cellNumbers":[8,9,12,13]},{"id":3,"action":"ACTION_ADD","type":"ANGLE_RIGHT_BOTTOM","result":7,"cellNumbers":[6,7,10]},{"id":4,"action":"ACTION_MULTIPLY","type":"L_HORIZONTAL_SHORT_LEFT_BOTTOM","result":24,"cellNumbers":[1,2,3,5]}]}
\ No newline at end of file
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-much-too-high.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-much-too-high.yml
new file mode 100644
index 00000000..92a6acf7
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-much-too-high.yml
@@ -0,0 +1 @@
+{"variant":{"gridSize":{"width":4,"height":4},"options":{"showOperators":true,"cageOperation":"OPERATIONS_ALL","digitSetting":"FIRST_DIGIT_ONE","difficultySetting":"EXTREME","singleCageUsage":"FIXED_NUMBER","numeralSystem":"Decimal"}},"savedAtInMilliseconds":1723717884585,"playTimeInMilliseconds":55612,"startedToBePlayed":true,"isActive":false,"cells":[{"cellNumber":0,"row":0,"column":0,"value":4,"userValue":4,"possibles":[]},{"cellNumber":1,"row":0,"column":1,"value":1,"userValue":1,"possibles":[]},{"cellNumber":2,"row":0,"column":2,"value":3,"userValue":3,"possibles":[]},{"cellNumber":3,"row":0,"column":3,"value":2,"userValue":2,"possibles":[]},{"cellNumber":4,"row":1,"column":0,"value":1,"userValue":1,"possibles":[]},{"cellNumber":5,"row":1,"column":1,"value":4,"userValue":4,"possibles":[]},{"cellNumber":6,"row":1,"column":2,"value":2,"userValue":2,"possibles":[]},{"cellNumber":7,"row":1,"column":3,"value":3,"userValue":3,"possibles":[]},{"cellNumber":8,"row":2,"column":0,"value":3,"userValue":3,"possibles":[]},{"cellNumber":9,"row":2,"column":1,"value":2,"userValue":2,"possibles":[]},{"cellNumber":10,"row":2,"column":2,"value":1,"userValue":1,"possibles":[]},{"cellNumber":11,"row":2,"column":3,"value":4,"userValue":4,"possibles":[]},{"cellNumber":12,"row":3,"column":0,"value":2,"userValue":2,"possibles":[]},{"cellNumber":13,"row":3,"column":1,"value":3,"userValue":3,"possibles":[]},{"cellNumber":14,"row":3,"column":2,"value":4,"userValue":4,"possibles":[]},{"cellNumber":15,"row":3,"column":3,"value":1,"userValue":1,"possibles":[]}],"selectedCellNumber":6,"invalidCellNumbers":[],"cheatedCellNumbers":[],"cages":[{"id":0,"action":"ACTION_ADD","type":"DOUBLE_HORIZONTAL","result":5,"cellNumbers":[2,3]},{"id":1,"action":"ACTION_ADD","type":"ANGLE_RIGHT_TOP","result":8,"cellNumbers":[8,12,13]},{"id":2,"action":"ACTION_MULTIPLY","type":"ANGLE_RIGHT_BOTTOM","result":4,"cellNumbers":[0,1,4]},{"id":3,"action":"ACTION_ADD","type":"SQUARE","result":9,"cellNumbers":[5,6,9,10]},{"id":4,"action":"ACTION_ADD","type":"L_VERTICAL_SHORT_LEFT_BOTTOM","result":12,"cellNumbers":[7,11,15,14]}]}
\ No newline at end of file
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-rating-matches.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-rating-matches.yml
new file mode 100644
index 00000000..0c8db59a
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-rating-matches.yml
@@ -0,0 +1 @@
+{"variant":{"gridSize":{"width":4,"height":4},"options":{"showOperators":true,"cageOperation":"OPERATIONS_ALL","digitSetting":"FIRST_DIGIT_ONE","difficultySetting":"EXTREME","singleCageUsage":"FIXED_NUMBER","numeralSystem":"Decimal"}},"savedAtInMilliseconds":1723800783333,"playTimeInMilliseconds":50719,"startedToBePlayed":true,"isActive":false,"cells":[{"cellNumber":0,"row":0,"column":0,"value":1,"userValue":1,"possibles":[]},{"cellNumber":1,"row":0,"column":1,"value":4,"userValue":4,"possibles":[]},{"cellNumber":2,"row":0,"column":2,"value":3,"userValue":3,"possibles":[]},{"cellNumber":3,"row":0,"column":3,"value":2,"userValue":2,"possibles":[]},{"cellNumber":4,"row":1,"column":0,"value":4,"userValue":4,"possibles":[]},{"cellNumber":5,"row":1,"column":1,"value":1,"userValue":1,"possibles":[]},{"cellNumber":6,"row":1,"column":2,"value":2,"userValue":2,"possibles":[]},{"cellNumber":7,"row":1,"column":3,"value":3,"userValue":3,"possibles":[]},{"cellNumber":8,"row":2,"column":0,"value":3,"userValue":3,"possibles":[]},{"cellNumber":9,"row":2,"column":1,"value":2,"userValue":2,"possibles":[]},{"cellNumber":10,"row":2,"column":2,"value":4,"userValue":4,"possibles":[]},{"cellNumber":11,"row":2,"column":3,"value":1,"userValue":1,"possibles":[]},{"cellNumber":12,"row":3,"column":0,"value":2,"userValue":2,"possibles":[]},{"cellNumber":13,"row":3,"column":1,"value":3,"userValue":3,"possibles":[]},{"cellNumber":14,"row":3,"column":2,"value":1,"userValue":1,"possibles":[]},{"cellNumber":15,"row":3,"column":3,"value":4,"userValue":4,"possibles":[]}],"selectedCellNumber":15,"invalidCellNumbers":[],"cheatedCellNumbers":[],"cages":[{"id":0,"action":"ACTION_NONE","type":"SINGLE","result":4,"cellNumbers":[1]},{"id":1,"action":"ACTION_NONE","type":"SINGLE","result":3,"cellNumbers":[7]},{"id":2,"action":"ACTION_MULTIPLY","type":"TRIPLE_VERTICAL","result":12,"cellNumbers":[0,4,8]},{"id":3,"action":"ACTION_MULTIPLY","type":"ANGLE_RIGHT_BOTTOM","result":12,"cellNumbers":[2,3,6]},{"id":4,"action":"ACTION_ADD","type":"ANGLE_RIGHT_TOP","result":7,"cellNumbers":[5,9,10]},{"id":5,"action":"ACTION_ADD","type":"ANGLE_LEFT_TOP","result":6,"cellNumbers":[11,15,14]},{"id":6,"action":"ACTION_SUBTRACT","type":"DOUBLE_HORIZONTAL","result":1,"cellNumbers":[12,13]}]}
\ No newline at end of file
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-too-high.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-too-high.yml
new file mode 100644
index 00000000..ce892ba8
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-easy-too-high.yml
@@ -0,0 +1 @@
+{"variant":{"gridSize":{"width":4,"height":4},"options":{"showOperators":true,"cageOperation":"OPERATIONS_ALL","digitSetting":"FIRST_DIGIT_ONE","difficultySetting":"EXTREME","singleCageUsage":"FIXED_NUMBER","numeralSystem":"Decimal"}},"savedAtInMilliseconds":1723716870483,"playTimeInMilliseconds":37638,"startedToBePlayed":true,"isActive":false,"cells":[{"cellNumber":0,"row":0,"column":0,"value":4,"userValue":4,"possibles":[]},{"cellNumber":1,"row":0,"column":1,"value":2,"userValue":2,"possibles":[]},{"cellNumber":2,"row":0,"column":2,"value":1,"userValue":1,"possibles":[]},{"cellNumber":3,"row":0,"column":3,"value":3,"userValue":3,"possibles":[]},{"cellNumber":4,"row":1,"column":0,"value":3,"userValue":3,"possibles":[]},{"cellNumber":5,"row":1,"column":1,"value":4,"userValue":4,"possibles":[]},{"cellNumber":6,"row":1,"column":2,"value":2,"userValue":2,"possibles":[]},{"cellNumber":7,"row":1,"column":3,"value":1,"userValue":1,"possibles":[]},{"cellNumber":8,"row":2,"column":0,"value":2,"userValue":2,"possibles":[]},{"cellNumber":9,"row":2,"column":1,"value":1,"userValue":1,"possibles":[]},{"cellNumber":10,"row":2,"column":2,"value":3,"userValue":3,"possibles":[]},{"cellNumber":11,"row":2,"column":3,"value":4,"userValue":4,"possibles":[]},{"cellNumber":12,"row":3,"column":0,"value":1,"userValue":1,"possibles":[]},{"cellNumber":13,"row":3,"column":1,"value":3,"userValue":3,"possibles":[]},{"cellNumber":14,"row":3,"column":2,"value":4,"userValue":4,"possibles":[]},{"cellNumber":15,"row":3,"column":3,"value":2,"userValue":2,"possibles":[]}],"selectedCellNumber":13,"invalidCellNumbers":[],"cheatedCellNumbers":[],"cages":[{"id":0,"action":"ACTION_ADD","type":"DOUBLE_VERTICAL","result":3,"cellNumbers":[8,12]},{"id":1,"action":"ACTION_ADD","type":"L_HORIZONTAL_SHORT_LEFT_BOTTOM","result":10,"cellNumbers":[0,1,2,4]},{"id":2,"action":"ACTION_MULTIPLY","type":"DOUBLE_HORIZONTAL","result":8,"cellNumbers":[5,6]},{"id":3,"action":"ACTION_ADD","type":"L_HORIZONTAL_SHORT_LEFT_TOP","result":10,"cellNumbers":[9,13,14,15]},{"id":4,"action":"ACTION_ADD","type":"L_VERTICAL_SHORT_LEFT_BOTTOM","result":11,"cellNumbers":[3,7,11,10]}]}
\ No newline at end of file
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-is-really-hard.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-is-really-hard.yml
new file mode 100644
index 00000000..fbdccab4
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-is-really-hard.yml
@@ -0,0 +1 @@
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\ No newline at end of file
diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-medium-too-high.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-medium-too-high.yml
new file mode 100644
index 00000000..5bac67f0
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-medium-too-high.yml
@@ -0,0 +1 @@
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diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-not-easy-rated-too-low.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-not-easy-rated-too-low.yml
new file mode 100644
index 00000000..1bf0f92d
--- /dev/null
+++ b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-not-easy-rated-too-low.yml
@@ -0,0 +1 @@
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diff --git a/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-schwierig-herausfordernd-gut-zu-spielen.yml b/gauguin-core/src/test/resources/difficulty-balancing/game_4x4-schwierig-herausfordernd-gut-zu-spielen.yml
new file mode 100644
index 00000000..6c72604b
--- /dev/null
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