In the final level of Heart of Gold, the player should destroy 5 inhibitors to lower the shield around The Duke. Destroying the inhibitors is listed as a required objective.
However, the player can detonate brimstone in the wall surrounding the duke, build stone bridges under the barrier, manually drop an imp on the fresh dirt, and then manually drop their army on the claimed tiles. This allows them to bypass all five inhibitors and immediately kill the Duke.
When the Duke is dead, the player can finish the level with the Rally flag, despite the Colossus being unable to reach the vault door.
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I will mention this here rather than it's own bug report, as I doubt it'll be changed. The final cutscene displays golden statues of all the significant cast from the HOG campaign. Mandalf's statue is present despite Mendeclaus' melting him in a cutscene at the beginning of level 3.
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