Please offer some advice.

Help, does anyone know? I'm playing "Heroes 4: Ring of Dragon," the campaign for Rin the Unrepentant, the second part of the campaign (I need to reach the elven city). To pass through the checkpoint, I need to stop the deforestation and defeat Marcus (the commander of the purple faction). I defeated all the purple troops, including Marcus, but they still won't let me pass through the checkpoint! I captured all the mines – nothing! There are 2 lumberjack huts under the purple flag behind the fence, along with peasants and dwarves, but I simply can't enter or even move the cursor there! What should I do? I've always completed all campaigns, but here I'm just wandering around the map and getting nowhere! Has anyone encountered this problem? Please help, I'd be grateful!