I suppose you could make 4 individual tree wall entities that are 1 tile each, and then simply arrange them in the right order to make them a tree.
E.g.
OTerrainMap
{
Pos = 0,0;
Size = 17,19;
Types
{
_ = 0;
# = 0;
p = GOLD wall(BRICK_PROPAGANDA);
$ = GOLD wall(BRICK_FINE);
^ = decoration(HOLY_TREE);
c = decoration(COUCH);
d = EBONY_WOOD door;
o = EBONY_WOOD wall(BRICK_FINE|WINDOW);
T = BIGTREE_WOOD treetopleft;
Y = BIGTREE_WOOD treetopright;
U = BIGTREE_WOOD treebottomright;
I = BIGTREE_WOOD treebottomleft;
}
}
{
.#o##o##o##o##o#.
##.............##
o...............o
#.....$$p$$.....#
#...$$$___$$$...#
#..$$_______$$..#
o..$___TY____$..o
#..$___IU____$..#
#..$_________$..#
o..$$_______$$..o
#...$$_____$$...#
#....$$___$$....#
o.....$___$.....o
#.....$___$.....#
#.....$___$.....#
#.....$___$.....#
o...............o
##.............##
.#o##o##d##o##o#.
And then just make the BIGTREE_WOOD material have such a high strength that it's nigh indestructable (and make it non-wishable too!)
Of course you couldn't have them spawned properly unless they were in pre-defined spots or you made one of those "random room" entries that contained only the tree.