I built a cellar with a grid of door frames. Later, stairs will lead up to the cellar. When the attackers enter the cellar, they will set off an array of vapor traps. I anticipate minimal damage.
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This reminds me of the Adoring Fan in Oblivion! 
Have you tested it against the Purge yet?
I haven’t, unfortunately. My purge meter keeps resetting to zero every time I log out of the game.
I just loaded my base up with melee and archer thralls. It is hard on the Purge since multiple angles of attack are employed. Have to do some repairs before I can fully test it out. I know it works the couple of times I have spawned NPC’s in and there is a pesky Leopard feeding in my base…