Dirty_Player
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Based on my experience playing TC3, having 18 castles in a starting setup doesn't change the core gameplay. In any case, it results in a limited selection of possible strategies.
As an alternative, you could simply make all secondary castles identical – weak castles, like Tower, Barbarian, and something else. This would balance the starting setup: there would be an option to play with a strong castle or a weak castle with a double bonus.