"Many evil things there are that your strong walls and bright swords do not stay. You know little of the lands beyond your bounds. Peace and freedom, do you say? The North would have known them little but for us. Fear would have destroyed them. But when dark things come from the houseless hills, or creep from sunless woods, they fly from us. What roads would any dare to tread, what safety would there be in quiet lands, or in the homes of simple men at night, if the Dunedain were asleep, or were all gone into the grave?" FotR:286.
How, with no spells?
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There are also at least two instances where Aragorn wakes up out of a sound sleep, somehow sensing that something is wrong... This "sounds" like the use of an Alarm spell, to me.
<snip>
'Come hither!' said Faramir. 'Look at me! Do you know the name of this place? Have you been here before?'
Slowly Gollum raised his eyes and looked unwillingly into Faramir's. All light went out of them, and they stared bleak and pale for a moment into the clear unwavering eyes of the man of Gondor. There was a still silence. Then Gollum dropped his head and shrank down, until he was squatting on the floor, shivering. 'We doesn't know and we doesn't want to know,' he whimpered. 'Never came here; never come again.'
'There are locked doors and closed windows in your mind, and dark rooms behind them,' said Faramir. 'But in this I judge that you speak the truth.'"