There are certain things that can prevent mobs from spawning within your base. Wards are a good one to use, and they can work for decorative purposes too, so you don't have to slap down workbenches all over the place. For more things you can use, I recommend this video: https://www.youtube.com/watch?v=_C9fKFg6bmw

But keep in mind, these only stop mobs from spawning INSIDE the base. They DO NOT prevent raids, which is what sounds like was going on when the two trolls attacked.