A battle for Taipei could hinge on defending this river. Here's how Taiwan's forces are readying for that fight.

A Taiwanese soldier opens fire with a machine gun on a Clouded Leopard infantry fighting vehicle.

Ann Wang/REUTERS While concerns about Taiwan's defense often center on beaches, one of its major vulnerabilities is a river.

The Tamsui River cuts into the heart of Taipei, where its government and military headquarters.

Taiwanese forces drilled earlier this month to defend it in multiple urban scenarios.

TAIPEI — In the dead of night, scores of masked military police moved a medley of defensive gear beneath flickering traffic lights and store signs.

Hemmed in by rows of shophouses, they laid metal barricades, land mines, and sandbags along an intersection just a mile from Taiwan's presidential office.

Behind them, Clouded Leopard infantry fighting vehicles prepared to roll out and open fire on incoming enemy forces.

This flurry of activity along a critical approach into the capital came toward the end of Taiwan's 10-day annual Han Kuang military exercise .

In this scenario, the simulated fight had made its way into central Taipei.

The blockaded roads led to the Guangfu Bridge, a four-lane crossing that spans the river dividing New Taipei City and the capital proper.

The Tamsui River, snaking through northwest Taiwan, was at the heart of many of this year's Han Kuang drills.

Although concerns about an attack by Beijing often center on a massive, D-Day-style landing across the Taiwan Strait, Taiwanese forces have spent considerable time practicing their defense of the Tamsui.

A Clouded Leopard armored vehicle is deployed during a drill simulating a response to curb invading forces from advancing further into Taipei.

Daniel Ceng/Anadolu via Getty Images The strategic waterway, fed by some of the island's largest tributaries, converges in the capital and then spills into the strait, where the Chinese province of Fujian lies some 100 miles across the water.