No. The word impregnable already means "impossible to enter."
You can replace the word impregnable with "impossible." If you still want to use the word, you can say
"The opposing fort proved impregnable during the battle."
Having the clause first is unwieldy.
We went out to battle the opposing army.
The Russians captured the embankment during the battle of Austerlitz.
Wolfe and Montcalm were the opposing commanders during the Battle of the Plains of Abraham. Both were killed in the battle.
The army had a stockade that surrounded the fort for protection during the battle.
Dak To was a major battle of the Viet War. For detailed information, see the website covering it. In the meantime, the 173rd Abn bde (Airborne Brigade) was the primary US Army unit during that fight.
A naval battle is when two opposing navies fight each other on the sea - usually during a war between two or more countries.
Detonating the opposing side in a war or battle.
The Hutu's and the Tutsi's
A battle line is a notional line on either side of which opposing forces are arranged in the military.
Because the pontoons that Burnside needed did not arrive for a few days, and Lee had time to man the high-lying positions so they were impregnable.
The Union Army & The Confederate Army
Conflicts between opposing forces.