I'm not a huge fan of this mission, honestly. I understand the concept, but limiting the defender to piecemeal deployments and restricting them in unit type seems really, really bad for the defender. I don't see any reason for the defender to place anything but craters, and the three pieces placed by the attacker will almost certainly be single-story buildings that are easy to destroy.
This is a great concept, but it needs to be playtested and rebalanced. It would be great if the defender could choose to concentrate their units (at risk of letting attackers freely move into undefended areas!) for example.