Northampton was important in medieval times as it was roughly halfway between York and the capital, London, and a stopping point on the Royal Progress. Henry I made the decision to hold parliament here in 1131. Parliaments were then held at Northampton Castle throughout the Early Middle Ages.
Early Parliaments weren't like today and began with the Curia Regis. The Magna Carta set the principles of modern parliament. The Commons were included for the first time in the reign of Edward I, although Commons and Lords didn't meet separately until 1341. The last Parliament at Northampton was on 5 November 1380 during the reign of Richard II. It was during this Parliament that it was decided a poll-tax of 3 groats per head would be charged to all people over the age of 15. It was this unpopular decision that led to the Peasant's Revolt in June 1381.
Parliament did still meet at other places outside London until after the Civil War in 1681. It never met in Northampton again, perhaps because during the last parliament there was not enough room to accommodate both Houses and their retinues. It was a wet winter and the roads were flooded in every direction meaning some of the nobles arrived after the start of Parliament and in a bad mood.
See also:
1131: Henry I calls parliament at Northampton Castle
1645: Battle of Naseby
1647: Charles I held prisoner at Holdenby House

