It's best known for the role it played in the English Civil War. Newbury was a strategically important town, so both sides wanted control of the castle. In 1643, King Charles I sent Sir John Boys and around 200 men to claim it.
By this time, Donnington was obsolete. The 250 year old walls were no match for modern artillery, so Sir John set to work improving the defences. He built earthworks in a star shape around the castle, creating gun emplacements and effectively turning the whole hilltop into a fortress.
You can still make out the shape of the 17th century defences.
Donnington came under attack in July 1644, when an army of 3000 Parliamentarian soldiers arrived. A major assault was launched up the hill, equipped with ladders to try and climb the walls. The attackers were driven back, losing more than 300 men.
Parliamentarian cavalry with the castle in the distance
After failing to capture the castle, the Parliamentarians tried to destroy it. They placed an artillery battery at the bottom of the hill, on the Newbury side. In a bombardment lasting 12 days, they demolished all of the towers on this side of the castle.
Despite the heavy damage, the defenders still had a strong position on the fortified hilltop. The situation became a stalemate, as the castle refused to surrender and the Parliamentarian soldiers refused orders to try another attack.
All the Parliamentarians could do was resume their artillery bombardment. After doing further damage to the front of the castle, they then set up cannons on Snelsmore Common to attack the other side.
The red patch on the closest tower shows where artillery damage has been repaired
By now, the building was falling apart. The Parliamentarians urged Sir John to save the castle by surrendering, but he replied that 'he was not bound to repair the castle, but that by God's help, he would keep the ground.'
Towards the end of 1644, two soldiers managed to sneak up and poison the castle well, which was about 400 yards outside of the walls. They probably thought their commander would be pleased. In fact, he was appalled to hear what they'd done and sent a warning to the enemy soldiers in the castle. A short truce was agreed, giving the defenders time to clean the poison out of their well.
As a result, the garrison were able to hold out until the war ended in 1646. Even then, Sir John only surrendered once he had permission from the King.
Later that year, Parliament decided to demolish what was left of the castle, although they left the gatehouse standing.
It's still an impressive sight today and has become one of Newbury's top tourist attractions.