Yep... I'm not good at playing on Castle at all; I pretty much avoid it at all costs.
Congrats on beating it on Impoppable... Kudos to you!
4-x Bloonchippers are crucial to late-game, if you ask me. Monkey Pirates are cheaper than First Strike Capability, but First Strikes are way more effective, since they can take down ZOMGs.
I'm a huge user of Apprentices and Villages. I've also experimented with Bloonjitsu-MIB Call For Arms, which turned out to be super-powerful until MOAB-classes arrived. Since the Bloonjitsus could pop lead with the upgrade, well, all non-MOAB class balloons suffered.
Apprentices are my way to go. Super Monkey Fan Clubs, paired with MIB Call For Arms, make a major bloon-destroying machine. Apprentices are my starting tower, though.
And then Battles comes and ruins everything. Because only three towers.
Apprentice-Buccaneer-Ninja. The Flash Bomb upgrade works perfectly with Dragon's Breath.
What's your main strategy?