The bombardment should be available when the battle begins. In a field battle waiting two minutes means you get one salvo if the enemy advances quickly. I don’t think I’ve ever managed two strikes. Once the rout begins I won’t risk hitting my own people.

Hitting a fortress during an assault can be useful if you don’t mind not having its attributes on the turn you capture the place. Any damage at all voids its strategic features so I almost always try to take a place intact. I have learned that if you blast open just one gate in an assault, if an enemy attacks before you can affect repairs then ALL of the gates will be open when you defend.

Strategic bombardments are also a way to get ships some experience if there is nothing else for them to do. Get cracking you slovenly landlubbers! Earn your keep!