A monitoring system (100) monitors displacement of a wind turbine tower and includes at least one plumb bob with an upper part and a lower part, each plumb bob being configured to be pivotally suspended at its upper part, via a suspension device, from a point above so as to attain a rest position in a rest situation, and each said plumb bob has one or more sensing surfaces (12, 12′). One or more suspension devices means (10) suspend the at least one plumb bob. Two or more sensors (14, 14′, 14″), each being configured to sense, in a specific sensing direction (16, 16′, 16″), a distance to a plumb bob, provide displacement data. At least two of the two or more sensors (14, 14′, 14″) are arranged in a sensing vicinity of a plumb bob, with at least two of the specific sensing directions (16, 16′, 16″) not being parallel to each other. The monitoring system includes a control unit (18) configured to receive the displacement data from two or more of the sensors, and a device for reporting, to an external unit (20), parameter(s) representing displacement of a wind turbine tower.