Keeping the ability to drop a war flag, and stopping the blocking/walling/grid-locking of Thingwalls approaches a softlock scenario;
I propose we add in Roots:
We add in Snake Like (Xylocarpu), Knee knobs (Bruguira), and/or pencil roots (Sonneratia) around the Thingwalls. Some roots would be walk-able by hearthling and critters alike. Roots would stop the ability of building or placing objects in some of the area around the tree. Some smaller roots nobs/knees could be little blockers that also spawn in the shallows. A few main roots could branch from the tree and fork out, allowing for a paths back to the Thingwall.
Edit:
Alternatively a thorny/thicket area could grow and iterate, at increasing intervals as we approach its challenge windows, dealing damage to objects around the Thingwall.
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Alarm Traps
Bard's Ring
Boreworm Dens
Boreworm Cuisses
Barrel Rack
Halberds
Merchant's Pack
Scaled Vests
Seasoned Gravy
Stone Castle Wall
Village Allies
Windmill Machines