La Serra, Medieval castle ruins in Grisons, Switzerland

La Serra is a ruined barrier wall and tower southeast of Zernez in the Engadin, on the old pass road above the Spöl. Of the original 130-metre wall running from the river up the slope, only scattered remains survive; the tower, with three keyhole embrasures on each floor facing the road, was secured and given a new roof in 1996–98.

La Serra was a barrier wall above Zernez, built to stop enemies advancing from the south over the Ofen Pass. No records of its construction survive; the Swiss castle map suggests the 13th or 14th century, or possibly the Swabian War around 1495. A report for Emperor Maximilian confirms Austrian troops demolished the clausen at Zernez in 1499. Around 1570 Ulrich Campell called it an old structure, and it was renewed around 1624 during the Bündner Wirren before being abandoned.

The name comes from the Romansh serrar (to lock, shut off), in the Vallader idiom. The tower held a watch garrison in wartime rather than a resident family, and the wall remnants adjoining it to the north probably belong to a later packhorse station. Three keyhole loopholes face the road on each floor.

The ruins lie southeast of Zernez, right beside the old pass road toward the Ofen Pass, so they can be reached on foot from the village or by car along that road. The tower was secured between 1996 and 1998 and given a new roof; the gate's position can no longer be determined, and only scattered remnants of the 130-metre wall survive.

The tower's three keyhole slits on each floor face the old pass road, the side enemies would have come from. A 1499 report to Emperor Maximilian records that Austrian troops "broke the clausen at Zernetz and Scharl to the ground". The tower was secured and given a new roof between 1996 and 1998.