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[GoblinCookie]

Something amazing happened in my game.  While the goblins were besieging my fortress and I had my dwarves lined out at the end of the trap-lined entrance ready to 'invite them in' and then something I could never foreseen happened.  Their trolls (and only their trolls) seemingly dug their way into my fortress from above, arriving it seems right in the middle of my duckpond where they started to massacre the ducks before heading into the rest of the livestock zone to attack various other livestock.  I had to redeploy my dwarf soldiers to the livestock pen in order to deal with the trolls, who were then easily slaughtered because they were scattered about chasing the panicking livestock.  Once the trolls had all been killed, the goblin soldiers who were nearly all mounted fled the area as fast as their steeds could carry them.

I can very much admire the cunning of the goblins, though I suspect they had intended their evil plan to kill civilian dwarves not ducks, the question is how did they manage to pull it off.  It is not anything special about trolls, because two of my ducks managed to make their way back out and have now gone feral. They did not dig their way in because no AI forces can presently dig through walls of any kind, not even building destroyers.  So my theory is that they simply climbed in, starting off grasping the trunk of willow tree above and then climbed down into the roots which they were able to do because the roots were exposed to the air on one side (the side that leads to my duckpond).  The ducks simply climbed back up the same way and took flight while the goblins were thanfully unable to follow the trolls because they were mounted. 

The reason would be that the climbing rules, for reasons that have to be with subterranean trees allow climbing from the tree trunk to the tree roots and back again as long as the tree roots underneath are exposed to the air.  This limitation is supposed to prevent people from climbing down into the roots of surface trees, but surface trees whose roots are exposed inadvertantly can be climbed down into from the trunk.  On the other side of the roots are a surface pond that is presently frozen over, perhaps the squares of the lake also count as open, however that the lake was frozen over means the trolls cannot have swam through the lake and then climbed through the willow roots. 

Does anyone know if this theory is correct and has anyone ever encountered this issue before?

[Shonai_Dweller]

Have you been felling trees just above your duck pond recently?
They may have knocked small holes in the ground that are bloody hard to spot (unless you're a troll). They could be getting in that way.

[PatrikLundell]

Mounted invaders! I've never gotten any, and doubted they even existed in this version.

I think Shonai_Dweller's theory is very plausible. One indication of a hole is an inexplicable log pile on the floor inside the fortress. Another one is finding grass growing indoors (although there is a bug that occasionally generates underground vegetation called just "grass"). I've found that covering the ground with a stockpile makes holes easier to see (at least with the Phoebus tileset) as holes result in gaps in the stockpile indication pattern (that was a side effect of using stockpiles holding nothing to block tree regrowth).

Ducks can fly, and thus can easily have flown out through a hole in the roof (and your ducks aren't actually feral; they're just on a walk-about).

I haven't heard of a root access issue before: roots just seem to be a modification to earth that reverts back once the tree is cut.

[GoblinCookie]

Have you been felling trees just above your duck pond recently?
They may have knocked small holes in the ground that are bloody hard to spot (unless you're a troll). They could be getting in that way.

Yes, you are correct.  There are about 5 small holes above my duckpond, what seems to have happened is that I removed their roots when I cleared out the duck pond area and then I must have removed the trees between now and a year ago when the last goblin invasion with non-mounted goblins happened since they did not use that means to enter my fortress last time.

New rule seems to be never to cut a tree whose truck is directly above your fortress 1st floor.  The stockpiles I found were unneccesery, instead all you have to is zoom to one z-level above the ground and you can see the holes as little blue squares.

[vanatteveldt]

Even easier: just don't use the first z-level, unless you're desperately short of soil

[PatrikLundell]

Yes, avoid the first level if possible. If you do use it, cover the area above it with a floor, paved road, or stockpile (with nothing enabled) to stop any trees from growing up there (it's at least possible for existing saplings to mature without room for roots otherwise). Carefully cut down the existing trees when you don't expect trouble or distractions (like change of season, arrival time for a caravan, full moon, etc) and cover the holes immediately.

[GoblinCookie]

Even easier: just don't use the first z-level, unless you're desperately short of soil

That is what I plan to do for a future fortress.

Yes, avoid the first level if possible. If you do use it, cover the area above it with a floor, paved road, or stockpile (with nothing enabled) to stop any trees from growing up there (it's at least possible for existing saplings to mature without room for roots otherwise). Carefully cut down the existing trees when you don't expect trouble or distractions (like change of season, arrival time for a caravan, full moon, etc) and cover the holes immediately.

Why would we need to go through so much bother?  Roots only appear if you have soil underneath the trees above, the problem is that if you remove the roots directly beneath a tree trunk it leaves a whole that non-mounted opponants can use to jump into your fortress.  The idea would be to leave the area directly above your fortress 1st level undisturbed surely?

[PatrikLundell]

Actually the hole appears only if you cut down the tree, but the problem is that DF is currently suffering from rampant junglification, so if you don't do anything about trees the map gets covered by them and the FPS takes a nose dive.

[Bumber]

Actually the hole appears only if you cut down the tree, but the problem is that DF is currently suffering from rampant junglification, so if you don't do anything about trees the map gets covered by them and the FPS takes a nose dive.

Trees sometimes spontaneously collapse during growth.