All bits in the Control A register and the Lock bit (LOCK) in the Status register (WDT.STATUS) are write protected by the Configuration Change Protection mechanism.

The Reset value of WDT.CTRLA is defined by a fuse (FUSE.WDTCFG), so the WDT can be enabled at boot time. If this is the case, the LOCK bit in WDT.STATUS is set at boot time, too.