I think there was a barricade right after the burning forest fire. So if the riders run through the forest, they are stopped by the barricade and get caught between a forest fire and enemy fire.

That's why the cannons were essential. With the barricade gone, the only problem becomes the burning forest, which they managed to overcome by following directions sent by the balloon about where it is safe to run through. Like you said, Ikta's forces are spread thin, so the fire isn't that comprehensive.

That's just my interpretation though. I watched this while drunk so I'm hazy on the details.