Which material makes up that tower?
The tower is made of brick.
    src        edge_attr  dst
0     1  to the right of   15
1     2       perched on    0
2     2               on    0
3     2   to the left of    8
4     3   to the left of   15
5     4  to the right of   10
6     8  to the right of    2
7     9  to the right of   13
8    10   to the left of    4
9    10   to the left of   11
10   11  to the right of   10
11   11  to the right of   13
12   12     decorated by   14
13   13   to the left of    9
14   13   to the left of   11
15   14       decorating   12
16   15   to the left of    1
17   15       perched on    0
18   15  to the right of    3