While attempting to crest a fortress there is often an error at the end of the crest.
Instead of taking the fortress you will receive one of the following two message.
Invalid Target
The rebel army has recaputed the fortress.
When the message invalid target was given, attempting to crest a second time worked.
When the message "the rebel army has recaptured the fortress" was given, a second attempt at cresting resulted in the same message. The fortress could not be crested.
On an additional note, we had killed the rebel commander defending the fortress and he respawned with the captains, i am rather sure he should not.
-Jana