There is no glitch Fadly, clearly an enemy spy sabotaged your gates so Mongols caught you unprepared.

Thats because;
* You had no unit deployment time.
* Your troops protecting the gates all around the city.
* Your gates opened to the enemy,
* Your troops had morale penalty (like in ambushes) so they routed real quick.

Those are certain symptoms of a city infected by an enemy spy. And it is like that since Rome Total War.