• 客室きゃくしつ — guest room
• 皇室こうしつ — Imperial Family
• 教室きょうしつ — classroom
• 浴室よくしつ — bathroom
• 温室おんしつ — greenhouse
• 寝室しんしつ — bedroom
• 退室たいしつ — leaving a room
• 室内しつない — indoor / inside the room

室 combines the radical 宀 (roof) with 至 (to reach). Originally, it meant a protected inner space in a house. Today it’s used for many kinds of rooms and indoor locations.

寝室しんしつで本ほんを読よみます。
I read books in the bedroom.

教室きょうしつに学生がくせいが十人じゅうにんいます。
There are ten students in the classroom.