You could have the lizardmen use their flying snakes to bypass the walls.

Dropping some bezerker types in the middle of town during an assault would be very disruptive.

Dropping a bunch of dudes on the top of an isolated wall tower might get them a foothold. Also, it forces the humans to man more of their defenses more fully, wearing them down.

Finally, if the lizardmen can see better in the dark than the humans, getting a bunch dropped on the gate house at night would be a good way to take it.