You should almost have a sign...
When planning the server settings we discussed caves, and building in special locations in general, and decided to let things go* with the special understanding that people that choose to build there are placing a big bulls eye on themselves of their own accord!
* We have seen many servers where this is outright banned, and we didn't really want to be heavy handed about it - our philosophy is to try and have as few rules as possible to keep the servers functioning smoothly. If that can be done without making a rule, all the better!
With 6X damage, I image you can make a rather tough nut!