chopping trees leaves backdoors

hey everyone, you may already know this but I just ran into an issue with chopping wood on the Z-lev above my fort. I didnt realize but apparently chopping trees down can leave holes in the ground, if you have dug out the square under it then creatures can fall/jump down and gain access to your fort without having to go through the gates. The best solution is to make sure your fort is at least 2 levels below where you are harvesting wood but if you already have an established fort and want to harvest wood from directly above it, you are still able to but I would recommend building a floor over the holes that are left to keep baddies out. You can tell if the tree left a hole by inspecting the chop site and checking for ground or open space.

<Cross post from /r dwarffortress101>