Remember, your party is but one small invasion compared to the 4 armies that have invaded the tunnels not to mention the other parties that have attacked gates.
This was a secret gate protected well by the undead themselves. This was not a gate in regular use or from the signs found even occasional use.
They have already lost their local troll and what is probably the better portion of the local garrison. I suspect this one poor goblin is an unhappy camper, even for a goblin.