When motorcycle riders want to escape from the confines of the New York-New Jersey metropolitan area, they often follow the Hudson River north into Rockland, Orange, Westchester and Putman counties. The lower Hudson valley offers a great mix of twisty, hilly, two-lane back roads that share the woodlands and rocky lake country with the Appalachian Trail.

On both sides of the river, two and four-lane routes follow the Hudson’s sweeping contours through the highlands and past Revolutionary War battlefields and many other historic sites.

Route 9W, Route 6, and the Palisades Interstate Parkway merge in the hills above the Hudson’s west bank near Bear Mountain Bridge. For those who have never ridden across this span, just passing through its storybook tollbooth is worth the $.75 — at least once. On the other side, travelers can turn left and head north along scenic Route 9D as it follows the Hudson’s eastern shore.

As part of the Hudson River Greenway Trail from Bear Mountain Bridge to Beacon, the two-lane Route 9D is a fun motorcycle ride that’s full of interesting sights and places to stop — natural and manmade. I thought I had seen them all. But, I recently learned that the steep embankments and woods along 9D about 12 miles north of the bridge hide one of the most startling landmarks in the lower Hudson River valley.

A fortress for an arsenal.

The dark, vine covered remains of a massive fortress called Bannerman Castle dominate a small, rocky island a few hundred yards from the Hudson’s eastern shore. Frank Bannerman VI built this edifice in the early 1900s — not as a home, but as an arsenal for his immense collection of surplus military weapons and equipment purchased from counties around the world.

Bannerman has been called the “Father of the Army and Navy Store.” At one time, his company was the world’s largest commercial buyer and seller of surplus military equipment ranging from cannons and gunpowder to bayonets and scabbards to hats and boots. The business was immense.

For example, at the close of the Spanish American War, Bannerman purchased 90 percent of all captured equipment. During the Russian-Japanese war, he filled an order for 100,000 saddles, rifles, knapsacks, haversacks, gun slings, uniforms, and 20 million cartridges, plus a shipload of other assorted military goods.

Cities throughout the United States purchased cannons and artillery pieces from Bannerman’s company, and then placed them in town parks as monuments commemorating our armed forces.

Bannerman needed a new, more secure place to store his extensive inventory — especially the huge quantities of volatile black powder and assorted cartridges and artillery rounds. In 1900, he purchased Pollipell Island in the Hudson about 35 miles north of Manhattan. He then spent the next 17 years building Bannerman Castle.

Bannerman’s Scottish heritage shines through

Bannerman personally designed the island’s buildings, docks, turrets, garden walls and moat in the style of old Scottish castles. In addition to warehouse space, Bannerman built a home on the island. Unique decorative touches throughout included biblical quotations cast into fireplace mantles, and a shield between the castle towers bearing a coat of arms embossed with a wreath of thistle, leaves, and flowers.

Historic artifacts tucked away included a table owned by General Washington and expedition equipment used by Admiral Perry on his trip to the North Pole.

The castle can be seen best when traveling by boat on the river or along the train tracks that parallel the riverbank. On the north and east sides of the warehouse, 4 1/2-foot high letters cast in concrete read “Bannerman Island Arsenal.”

In 1967, the family sold the castle to New York State, which took possession after the Bannermans removed the remaining military gear and gave many historic relics to the Smithsonian.

Fire destroys the castle.

The state planned to open the castle and grounds as a park, but on August 9, 1969, a fire of unknown origin gutted and severely damaged the fortress and other buildings. Since then, the parks commission has declared the island and castle off-limits to the public.

During the late fall, I took a motorcycle ride up 9D to see the castle for myself. A few miles north of Cold Spring, I spotted the pedestrian overpass at Metro North’s Breakneck Ridge train stop. After parking the bike in a dirt lot on the east side of the road just south of the overpass, I walked about 1/16-mile up the road. A steep path up the embankment goes into the woods and leads to the overpass.

A giant chess piece on an island

I topped the embankment, cleared the trees and looked north out into the river. And there stood the ruins of Bannerman Castle. Looking like a giant, ornately carved chess piece misplaced on a tree-covered island. While generally rectangular, the castle is replete with curved spires, turrets and alcoves. There are no right angles. From a distance, it has the rippling, textured look of corduroy.

Through the bare trees, I could make out other smaller buildings near the water’s edge and on top of the island’s gradually sloping hill. A turret-like structure marked the outer edge of a small harbor, and two more like it sat in the water just off the west bank.

The whole scene, even on a bright sunny day, was like an apparition. After all, this is the Hudson River in New York, not Loch Ness in Scotland. I can only imagine how imposing and otherworldly Bannerman Castle must appear when seen close up through the fog and rain from a boat on the river.

To get an appreciation for the castle and its unique architecture and setting, click onto the Bannerman Castle Trust’s website at http://www.bannermancastle.org

The trust was formed to work with the state and other organizations to preserve the remaining structures on the island through a combination of volunteerism and fund raising. Perhaps, someday, the group can return this amazing fortress to its original condition and New York will open it again for all to visit.