I don't have much to go on this since I did not save the game.
After activating the two levers to lower the bridge, the bridge was in a lowered state, the sub-objective was accomplished and gone. However, none of my units could cross it. They were flailing away at it as if it was a solid barrier. At 750'000HP and always regenerating there was nothing I could do.
Only a few Occoluses and Bafus could get into the Emperor's throne room. 4~5 units at level 10 took forever to bring him down while my whole army was locked outisde and watching. After killing him I got the cutscene and 'my' 'victory'.
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