From the Wikipedia page 
The First Battle of St Albans, fought on 22 May 1455 at St Albans, 22 miles (35 km) north of London, traditionally marks the beginning of the Wars of the Roses. Richard, Duke of York and his ally, Richard Neville, Earl of Warwick, defeated the Lancastrians under Edmund, Duke of Somerset, who was killed. York also captured Henry VI, who appointed him Constable of England.
The Lancastrian army of 2,000 troops arrived at St Albans first, and proceeded to defend it by placing troops along the Tonman Ditch and at the bars in Sopwell Lane and Shropshire Lane. The 3,000-strong Yorkist army arrived and camped in Keyfield to the east. Lengthy negotiations ensued with heralds moving back and forth between the rival commanders. After several hours, Richard, despairing of a peaceful solution, decided to attack. The bulk of Henry's forces were surprised by the speed of Richard's attack; most of the army was expecting a peaceful resolution similar to the one at Blackheath in 1452. However, two frontal assaults down the narrow streets against the barricades made no headway and resulted in heavy casualties for the Yorkists.
Warwick took his reserve troops through an unguarded part of the town's defences, through back lanes and gardens. Suddenly the Earl appeared in the Market Square where the main body of Henry's troops were talking and resting. There is evidence they were not yet expecting to be involved in the fighting, as many were not even wearing their helmets. Warwick charged instantly with his force, routing the Lancastrians and killing the Duke of Somerset. Popular legend has it that he was killed outside an inn called The Castle, fulfilling a soothsayer's warning years earlier to beware of castles.
On the Earl's orders, his archers then shot at the men around the King, killing several and injuring the King and the Duke of Buckingham. The Lancastrians manning the barricades realised the Yorkists had ouflanked them, and fearing an attack from behind abandoned their positions and fled the town.
The First Battle of St Albans was relatively minor in military terms, but politically was a complete victory for York and Warwick: York had captured the King and restored himself to complete power, while his rival Somerset and Warwick's arch-enemies Henry Percy, 2nd Earl of Northumberland, and Lord de Clifford both fell during the rout.