The most effective method for keeping one's base & defensive fortifications safe from any and all invasion events is to drop everything you are doing and run outside your base beyond your defences. Enemies will almost always engage your character immediately, although because the detection range of trolls is so small, do not run too far away from them. They will head for your base/defences if agro is lost, but usually a well placed arrow shot is enough to gain their attention. I have dug a donut-shaped killing-pit, where I engage invasion enemies. It is simply a circular pit with a raised centre. Find a hill outside your base (but not so far that it is outside the usual red raid event circle on the mini-map) and dig a trench around it leaving the earth in the middle untouched. This will create a raised central area where you can pick off creatures from range, that are in the trench or circling on the outer edge. You may want to flatten the central hill and add some stakewall to stand behind as defence against ranged enemies. I cannot stress how important it is to run out of the base as quickly as you can once the invasion begins, as enemies will attempt to get to your location, destroying anything they can between them and you.