Hrothgar has many warriors sleep there at night to try to guard the "Great Hall" from being raided again, but this always fails. Each night, more soldiers are slaughtered and Hrothgar puts more in their places; however, this does not work because those are also killed and the pattern keeps repeating itself. eNotes adds:
Grendel haunts the hall by night for twelve years. The Danes despair of ridding themselves of him. They can neither defeat him nor come to terms with him.
Because they cannot defeat him, they become exasperated. Beowulf hears of their troubles and decides to come save them from Grendel.