Imagine a planet - henceforth Planet X - that is orbiting very close to a black hole. The space-time distortion will be very high there and according to the theory of relativity, the time there will pass slower than for the rest of the universe.
For those who have watched the movie Interstellar, it's similar to the first planet the mission visits (the one close to the Gargantua black hole).
In order to make the things more visual, let us assume a planet where the distortion is so strong that we have the next equivalence:
1 hour in Planet X = 10 years in the rest of the universe.
Now imagine the Earth sends several missions to Planet X, 1 every 10 years for 100 years.
The planet will receive those missions within the same day. The first mission to arrive will have to wait only 1 hour to meet the second mission that departed from Earth 10 years later.
Therefore, we see here that Planet X is very vulnerable in the way that a potential enemy could gather many "ships" and "troops" there in only a few hours, while actually taking decades to build that army. This way, the planet would be easily invaded.
However, the enemy would need a lot of time in order to invade Planet X, so much that it would not make sense to actually do it. Any resources on Planet X would need many years to be exported to the enemy planet.
What do you think about this perspective?