Not that rare. Superior forces, element of surprise, element of shock, a plan of attack made up, some insiders, etc. 
In medieval style city captures, they are either really quick, in where they overwhelm the defenses quickly, taken through breaching by way of siege, or starved out. 

The biggest problem the Otheans had here is that they were wholly unprepared, with mostly ceremonial guards. The real soldiers of Othea (beyond the small number of royal bodyguards) are at the borders. The Kalesians bypassed the border entirely. We imagine that when there are encounters later between the Othean Army and the Kalesian Army, provided the sides are roughly balanced in size and makeup, the rate of victory for the Kalesians will be roughly 50%.