Herot or Heorot Hall was commissioned by King Hrothgar to be built as a mead hall for his warriors. In the poem, the mead hall is called the "greatest of hall-buildings." It is where King Hrothgar fellowships with his warriors; during feasts, he lavishes the warriors with priceless gifts and treasures for their allegiance to him and for their courage on the battlefield.
The hall itself is tall, imposing, and immense. It is built sturdily, its exterior character mirroring the strength of warriors within: "...towered the hall up High and horn-crested, huge between antlers: It battle-waves bided, the blasting fire-demon..." The hall is a place of celebration and merriment. The warriors drink mead, tell heroic stories, and listen to the songs of bards ("there was dulcet harp-music, Clear song of the singer").
It definitely is a happy place to be. When Beowulf defeats Grendel, King Hrothgar presents him with gifts at Heorot: eight horses with gold-plaited bridles, an embroidered banner, a new helmet, chain-mail, and a new sword. Even the other warriors receive lavish gifts for their part in defeating Grendel. So, Heorot presents as a place that generally exhibits a positive atmosphere.
In contrast to Heorot, Grendel's lair is miserable, loathsome, and dark. Grendel is said to dwell in the moor-fens. Fens are wetlands, created by decaying matter in wet terrain. So, a fen is definitely a dismal place to live, quite unlike Heorot Hall. Grendel is so accustomed to living in the cheerless moors that he finds the merry atmosphere of Heorot almost unbearable. In the poem, we are told that the moor-fens are "mist-covered." It is always dark there; it is uninhabitable by humans and not readily accessible. Grendel and his mother live where wolves roam, where the air is unhealthy, and where the weather is always unpleasant.
They guard the wolf-coverts,
Lands inaccessible, wind-beaten nesses,
Fearfullest fen-deeps, where a flood from the mountains
’Neath mists of the nesses netherward rattles,
The stream under earth: not far is it henceward
Measured by mile-lengths that the mere-water standeth,
Which forests hang over, with frost-whiting covered,
A firm-rooted forest, the floods overshadow.
Uncanny the place is:
Thence upward ascendeth the surging of waters,
Wan to the welkin, when the wind is stirring
The weathers unpleasing, till the air groweth gloomy,
To thee only can I look for assistance.
And the heavens lower.