Besides the monastery, there was a castle of great strength, which often resisted the invader's hostile attacks; and heathen Danes had again and again been driven back to their ships by its stalwart garrison.  Its glories, however, are departed.  We could find only a few low walls, over which we could leap, and the remains of a staircase of eight or ten steps in a tower but little higher than the wall.  A board warning off trespassers took away what little romance we had conjured up.