Glyph of Fortification

The Glyph of Fortification, commonly shortened to Glyph, is an ability activatable by any player on a team that causes all friendly buildings to receive a massive armor bonus and magic immunity for effective invulnerability to all sources of incoming damage. The effect has a short 5 second duration with a 5 minute cooldown.

Mechanics
The Glyph of Fortification can be activated by any player on a team by a small icon in the bottom right corner of the screen as long as it is off cool down. When activated, it grants a massive impenetrable armor bonus to all buildings belonging to that team, including towers, barracks, and the Ancient, making them immune to any source of incoming damage. A team's Glyph cooldown is instantly refreshed when they lose a Tier 1 tower.

Despite being invulnerable to damage, units can still perform the action of attacking or targeting buildings under the effects of the Glyph.

Trivia

 * In earlier versions of DotA, Glyph did not grant magic immunity, and skills such as Pugna's Nether Blast could attack through the invulnerability.