Focusing on the openness and light:
* Thicket: A dense, tangled growth of bushes or small trees.
* Jungle: A dense, tropical forest.
* Undergrowth: A dense growth of low-lying plants.
* Woods: A dense forest.
Focusing on the beauty and pleasantness:
* Wasteland: A barren, desolate area.
* Swamp: A boggy, marshy area.
* Desolate: A barren, deserted, or lonely place.
Focusing on the specific purpose of a glade:
* City: If the glade is a place of peace and tranquility, a bustling city would be an antonym.
* Battlefield: If the glade is a place of beauty and serenity, a battlefield would be an antonym.
Ultimately, the best antonym for "glade" depends on the specific context in which you are using the word.