I have 2 donut type killing pits outside my major bases to the north & south. Basically just a big round pit with a high built up area in the middle where I run to when an invasion event begins. By standing on the raised dais in the middle I am able to kill enemies at will. Since I did this I have not died during an invasion, nor have any of my structures or tamed animals sustained any damage.

Find a very large rock or hillock and dig a deep moat around it. This way there is no need for levelling or building which can reduce frame-rate and impede game performance.