This would be a shield (possibly a mines version as well) that provided defense proportional to the level of the person attacking it. Should be a limited structure (maybe 2 per world max). Something like

Reactive Barrier

When the planet is attacked, provides defense equal to 3X the level of the attacker.

It'd be helpful in warding off high level attackers, especially if one couldn't build obviation barriers yet. But of course, against a level 300 invader, the obviation barrier would be better - thus a choice between a guaranteed medium of defense, or something more flexible.

With a limited number per world (and if it was a mission reward, a limited number, period), it wouldn't be an insurance policy against a high level attack, but could provide some extra oomph to ward off an invasion. At the same time, the shield would never be obsolete - it would maintain its usefulness even as more players reach higher levels. Plus, since it scales with low levels too, it's a shield that would not be impregnable by lower players, as it would affect them much less.

something that powerful would have to have a very limited supply