Qantara, Egypt - A military garrison of mud-brick and seashells unearthed in Egypt's Sinai desert may be key to finding a web of pharaonic-era defenses at the northeast gateway to ancient Egypt, archaeologists said on Thursday.

Archaeologists who discovered the 3,500-year-old garrison, where up to 50,000 soldiers could be posted in times of heightened tensions, say they hope inscriptions at Luxor's Karnak temple may serve as a guide to finding other outposts.

But knowing the location of the garrison at the ancient city of Tharu, in a formerly fertile area of Egypt where a branch of the Nile river once met the Mediterranean Sea, is key to understanding where to start looking.

"As we understand from the inscription at Karnak temple, the city of Tharu had two fortifications with the Nile in the middle," said Mohamad Abdul Maqsoud, who heads archaeological exploration in Egypt's Nile Delta and Sinai regions.

In their 3,000-year history, Egypt's Pharaohs often ventured across Sinai to fight Hittites and other civilizations in the area now covered by Israel, the occupied Palestinian territories, Lebanon, Jordan, Syria and Iraq.

"This city was used to protect Egypt and as a gate to the Delta. It was a post of control. If you wanted to cross the Nile, you asked for permission before you crossed the bridge," Abdul Maqsoud said.

Most Egyptian fortifications at the time were made of stone, not easily available in the Sinai. So Egyptians used seashells to strengthen the mud brick used to build the garrison, with a 15 meter thick and 12 meter high wall to discourage attack.

Garrison Used To Expel Hyksos

Abdul Maqsoud said a full exploration and excavation of possible pharaonic military installations in Sinai could take 15 years to complete, but he expected to find more outposts. The garrison, called Tell Heboua, is now some 15 km (9 miles) from the coast and the Nile branch is no longer there.

The fortress was first used as a base from which to expel the occupying Hyksos, who occupied Egypt for some 120 years between 1620 and 1534 BC and whose capital, Avaris, was located nearby in the Delta, Abdul Maqsoud said.

Their expulsion is seen as the beginning of the New Kingdom.

"When the Egyptians liberated Egypt it was a very important military action against the city here by the King Ahmose I," Abdul Maqsoud said.

The fortress later played a significant role in military actions against the Hittites during the reign of Ramses II during the 19th pharaonic dynasty.

Abdul Maqsoud said some 30,000 civilians lived in the adjacent city, which was also a major trading post, and the site contains human and crocodile remains, remnants of temple columns and broken pottery.

"They sold very good wine from this area in the pharaonic time. This is described even in the tomb of Tutankhamun," Abdul Maqsoud said.

Abdul Maqsoud said Frenchman Ferdinand de Lesseps discovered a large inscribed stone in the area during construction of the Suez Canal in the 1850s and diverted the canal's path to preserve the site.