We are told that he

" ....had reigned for years a score" (line 1)

A score is 20, so he ruled for 20 years. 

It was a hard period of time for the country.  There was

"Battling, struggling, pushing, fighting, killing much, and robbing more" (line 2)

During the poem, he relates he has fought over land and sea, and no king knew more glory than he did. But now he is sick and tired and having horrible nightmares of the things he did in the past.  There were

"Cities burning, convents blazing, red with sacrilegious fires; Mothers weeping, virgins screaming vainly for their slaughtered sires."  (Stanza 11)

The Bishop tries to sooth him by telling him of all the good things he has done. 

"Look, the land is crowned with minsters, which your Grace's bounty raised." (Stanza 13)

A minster is a church in England.  So he has built many churches.  We know that King Canute was a very religious king. 

"Abbey's filled with holy men, where you and Heaven are daily praised."

There are also many religious abbeys where men are training or have become priests.  They pray for the King daily.