The ancient tower has leaned several degrees off vertical due to centuries of foundation settlement.

leaned 是动词 lean 的过去分词形式。lean 本意是"倾斜、倚靠"，这里表示建筑物因地基沉降而发生的倾斜状态。off vertical 表示"偏离垂直方向"，强调倾斜的程度。