A bit rushed. Basically she's offering two options.

One: is after you sack a fort, village, or what ever that's near a river. You rip apart everything, and throw it into the river to fill it and build a damn. Including the bodies.

Two: After taking over a village or town, you force the villagers to build the bridges by salvaging buildings and the likes for material, and carry them.

My thought is... it wouldn't be a good idea to let a trolloc build anything complicated that needs to hold a lot of weight.