There was no movement of air down under the forest-roof, and it was everlastingly still and dark and stuffy.

Even the dwarves, the kings of the underground, felt sufficated.

Supernaturally thick cobwebs, stretched from tree to tree.

Night is pitch black.

Extremely large spiders reside in the forest.

Below is a more intense description.
(All passages taken from The Hobbit unless otherwise noted)

The entrance to the path was like a sort of arch leading into a gloomy tunnel made by two great trees that leant together, too old and strangled with ivy and hung with lichen to bear more than a few blackened leaves.
The path itself was narrow and wound in and out among the trunks. Soon the light at the gate was like a little bright hole far behind, and the quiet was so deep that their feet seemed to thump along while all the trees leaned over them and listened. As theft eyes became used to the dimness they could see a little way to either side in a sort of darkened green glimmer. Occasionally a slender beam of sun that had the luck to slip in through some opening in the leaves far above, and still more luck in not being caught in the tangled boughs and matted twigs beneath, stabbed down thin and bright before them. But this was seldom, and it soon ceased altogether. There were black squirrels in the wood. As Bilbo's sharp inquisitive eyes got used to seeing things he could catch glimpses of them whisking off the path and scuttling behind tree-trunks. There were queer noises too, grunts, scufflings, and hurryings in the undergrowth, and among the leaves that lay piled endlessly thick in places on the forest-floor; but what made the noises he could not see. The nastiest things they saw were the cobwebs: dark dense cobwebs with threads extraordinarily thick, often stretched from tree to tree, or tangled in the lower branches on either side of them. There were none stretched across the path, but whether because some magic kept it clear, or for what other reason they could not guess.

It was not long before they grew to hate the forest as heartily as they had hated the tunnels of the goblins, and it seemed to offer even less hope of any ending. But they had to go on and on, long after they were sick for a sight of the sun and of the sky, and longed for the feel of wind on their faces. There was no movement of air down under the forest-roof, and it was everlastingly still and dark and stuffy. Even the dwarves felt it, who were used to tunnelling, and lived at times for long whiles without the light of the sun; but the hobbit, who liked holes to make a house in but not to spend summer days in, felt he was being slowly suffocated.

The nights were the worst. It then became pitch-dark - not what you call pitch-dark, but really pitch; so black that you really could see nothing. Bilbo tried flapping his hand in front of his nose, but he could not see it at all. Well, perhaps it is not true to say that they could see nothing: they could see eyes. They slept all closely huddled together, and took it in turns to watch; and when it was Bilbo's turn he would see gleams in the darkness round them, and sometimes pairs of yellow or red or green eyes would stare at him from a little distance, and then slowly fade and disappear and slowly shine out again in another place. And sometimes they would gleam down from the branches just above him; and that was most terrifying. But the eyes that he liked the least were horrible pale bulbous sort of eyes. "Insect eyes" he thought, "not animal eyes, only they are much too big."

Although it was not yet very cold, they tried lighting watch-fires at night, but they soon gave that up. It seemed to bring hundreds and hundreds of eyes all round them, though the creatures, whatever they were, were careful never to let their bodies show in the little flicker of the flames. Worse still it brought thousands of dark-grey and black moths, some nearly as big as your hand, flapping and whirring round their ears. They could not stand that, nor the huge bats, black as a top-hat, either; so they gave up fires and sat at night and dozed in the enormous uncanny darkness.

 

He had picked his way stealthily for some distance, when he noticed a place of dense black shadow ahead of him black even for that forest, like a patch of midnight that had never been cleared away. As he drew nearer he saw that it was made by spider-webs one behind and over and tangled with another. Suddenly he saw, too, that there were spiders huge and horrible sitting in the branches above him, and ring or no ring he trembled with fear lest they should discover him. Standing behind a tree he watched a group of them for some time, and then in the silence and stillness of the wood he realised that these loathesome creatures were speaking to one another.... With that one of the fat spiders ran along a rope, till it came to a dozen bundles hanging in a row from a high branch. Bilbo was horrified, now that he noticed them for the first time dangling in the shadows, to see a dwarfish foot sticking out of the bottoms of some of the bundles, or here and there the tip of a nose, or a bit of beard or of a hood.

"Whatever. Legolas has a palace, doesn't he?"
Not quite -- but his dad, Thranduil (King of Mirkwood) has a "hall".

In a great cave some miles within the edge of Mirkwood on its eastern side there lived at this time their greatest king. Before his huge doors of stone a river ran out of the heights of the forest and flowed on and out into the marshes at the feet of the high wooded lands. This great cave, from which countless smaller ones opened out on every side, wound far underground and had many passages and wide halls; but it was lighter and more wholesome than any goblin-dwelling, and neither so deep nor so dangerous.

These are not wicked folk. If they have a fault it is distrust of strangers. Though their magic was strong, even in those days they were wary. They differed from the High Elves of the West, and were more dangerous and less wise. For most of them (together with their scattered relations in the hills and mountains) were descended from the ancient tribes that never went to Faerie in the West. There the Light-elves and the Deep-elves and the Sea-elves went and lived for ages, and grew fairer and wiser and more learned, and invented their magic and their cunning craft, in the making of beautiful and marvellous things, before some came back into the Wide World. In the Wide World the Wood-elves lingered in the twilight of our Sun and Moon but loved best the stars; and they wandered in the great forests that grew tall in lands that are now lost. They dwelt most often by the edges of the woods, from which they could escape at times to hunt, or to ride and run over the open lands by moonlight or starlight; and after the coming of Men they took ever more and more to the gloaming and the dusk. Still elves they were and remain, and that is Good People.

He imprisoned a member of Bilbo's party on false assumptions (the king thought his feast had been attacked, when in reality the party was merely begging for food), and even after the prisoner explains the situation, he fails to explain why he was in Mirkwood to begin with. This angered the king, and so he stated: "Take him away and keep him safe, until he feels inclined to tell the truth, even if he waits a hundred years."