From within the workbench, you can choose to be attacked by a variety of enemy groups in order to "test" your settlement. There should be a risk and reward. The risk, obviously, is the destruction of your settlement. The reward could be caps, XP, or unique gear.
Choosing to be attacked by synths, for example, you can further choose their level of attack. For example, a tier one attack would be just a few synths beaming in randomly. The next tier might be a half-dozen synths and a courser. A tier 3 might be an attack on par of the castle attack towards the end of the main storyline. A tier 4 would be a massive attack involving a great risk at the loss of all within the settlement, but also come with the greatest rewards.
Choosing to be attacked by super mutants, a tier 3 may involve a behemoth as well as a horde of super mutants coming from all directions.
To stretch this DLC out a little further, you can scatter magazines throughout the Commonwealth that could allow for different enemy attacks… For example, a horde of deathclaws , or a horde of mirelurks including a queen.
I imagine that the toughest of all the test attacks would be a gunners attack. A tier three gunner attack would include at least two assaultrons along with wave after wave of gunners coming from all directions, and a tier four would just be a huge pile of holy crap followed by a day curled up in fetal position babbling on about bad decisions. Bring it on!