Undead could use a home bridge which would have stats like this.
Bone bridge
Desc: a bridge made of bones.
Turn/build:2
Hp:6
Sight:1
Arm:0
P.arm:6
Probably let it leave a corpse on destruction to give it some unique taste.
Ruin bridge:
Desc: A dark, ominous bridge.
Turn/build:4
Hp:18
Sight:1
Arm:1
P.arm:20