Tyrant

They have blocked the space, which they had no need to do, seeing it is extremly hard to break the space in Devil's Gate. Elite Tyrants could not do it; only Leader class Tyrant with exceptional capabilities could do it.

They likely feared that a Leader class Tyrant might come in for the rescue, which is likely to happen. Each leader class Tyrant is very busy, and if it is not something extremly important, they did not take action.

Leaders are precious; each of them is equal to the hundred Elites, and losing a single one could affect war results.

Boom Boom Boom 

Fighting had already seemed to have begun, and if sounds were any proof, it is going intense. If I want to leave, it would be a great time for leaving; if battle progressed further or some Grimm Monster was able to break into the Airship, it would be too late.