If you attack multiple castles at the same time, you do NOT send the same amount as the next capture. If you do this, you may not capture all your castles.
For example, if you are attempting to capture your 3rd and 4th castle separately, you would need 5k total.
3rd castle - 2k
4th castle - 3k
If you attempt to capture both castles at the same time you would need to send 3k to each castle. The reason being, you don't know which castle will fall first so you have to hedge bets and send 3k to each.
Make sense?