[Hitchhiker 42]

Originally Posted by Van.Augur

Time to play devil's advocate:

If you want to be exact, you might want to take into account that the rough terrain, according the rules, is a random layout of the steel cubes. There are an extremely large number of rough terrain defenses that could theoretically exist (keep in mind they aren't symmetrical when you rotate them 180 degrees), making the real number of possible unique defense layouts impractically large...
but still calculable

Assuming, this, which it isn't (as per above post), you'd be completely limited by the number of built rough terrains.