Breakable Glass blocks off an area until it's smashed.

Mask a wall and texture it with breakable glass (#503). Use T to make the glass transparent. Press B and H on it (or B and Ctrl + H in 2D mode) to make it blocked and hittable.

Notes:

• Breakable glass without blocking/hitscan flags enabled can still be shattered by a nearby explosion. If you don't want to use other glass textures, and want to make this specific texture unbreakable, apply the texture to sprites instead.
• Breakable glass walls with one-sided masking (Shift + M) and blocking/hitscan flags enabled can be shot from the invisible side, leaving bullet holes in mid-air. If you then shatter the glass from the proper side, the bullet holes will remain.