Only use he ballistas when on the attack otherwise the slow reload time wil bite you in the back because you could have used that space for a better unit. Let me explain why only on the attack: When you defend the enemy comes to you, and you artillary cannot fire many shots. When attacking the enemy waits for you to come to them and then you can murderlize them with your ballsistas. Beware of the enemy having artillary as they can knock yours out.