I wanted to say this colonel is the most unbelievable thing in this chapter, but story has too many examples of high commanding officers doing exactly this because they thought the enemy was weak and ignoring blatantly obvious details, that aren't even details sometimes, more like the whole thing of the situation.
For those having problems understanding the situation, here a little simplified summary ( it is mostly my assumptions ), the girl that is the leader of the students is from an annexed country of Albion her reason may be political or personal ( there are hints of personal ), she is working with Castor that wants to use this whole confusion to demoralize the queen reign and usurp the throne he is working with Dhala and what Dhala wants is pretty obvious, destabilize their enemy country reign supreme over the world yadda yadda yadda blah blah blah, you know... the basic, not different from Albion truth be told.
The military-speak isn't all that, the fortress is star-shaped and the entrances are in the concave part so an invading army always has to face two walls which make easy for defence, the colonel is obviously only there to complicate things but i think he was trying for real to kill the students, but he is an idiot and forgot that they are military cadets and the fortress is ancient for a reason.