Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Name | Date | Condition | Ownership\n/ Access | Notes |
| --- | --- | --- | --- | --- |
| Château d'Angers | 13th century | Substantially intact | City of Angers | Walls nearly 2,000 feet (610 m) in circumference, incorporating 17 towers originally 130 feet (40 m) high. |
| Château de Baugé | 15th century | Intact |  | Built as hunting lodge. |
| Château de Bourmont | 16-19th century | Intact and extended | Private | Extended in Neo-Gothic style |
| Château de Brissac | 15-17th century | Rebuilt | Private | Damaged during French Wars of Religion, rebuilt 1611 in baroque style. |
| Château de Champtocé | 13-16th century | Ruins |  |  |
| Château de Montreuil-Bellay | 11-13th century | Substantially intact | Private | Divided into rental units in 1822, restored after 1860. |
| Château de Montsoreau | 1455 | Restored | Département | Ruinous by late 19th century, restored, houses the Musée des Goums Marocains. |
| Château du Plessis-Bourré | 1468-1472 | Intact | Private (open to the public) | Externally unchanged since the 15th century, working drawbridge. |
| Château du Plessis-Macé | 13-16th century | Intact |  | Converted to house 15th century. |
| Château de Pouancé | 12-13th century | Ruins |  | Considered second fortress of Anjou, after Angers. |
| Château de Saumur | 12th century | Restored | City of Saumur | Page for September in the Tres Riches Heures du Duc de Berry depicts the Chateau as it looked in 1410. |
| Château de la Turmelière | 13th century | Ruins |  | 19th century building of same name nearby. |
Question:
when was the last external change made on château du plessis-bourré?
Answer:
15th century