וַיִּשְׁפְּכוּ סֹלְלָה אֶל הָעִיר

As part of their plan of attacking the city of Aveil, Yoav's men created a ramp to get over the wall. This is a very ancient war tactic. Below is a picture of a dirt ramp the Roman's made in order to get into Masada (it is the big pile of dirt on the lower right hand side of the pictures that acts like a bridge).