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Some siege ideas, and battle ideas, concerning defense and capture.
This idea comes from capturing but it's taken to another level.
The idea is, some buildings must have a requirement before been captured ( or process to start capturing.
The idea is "a building is captureable if you you fill some condition to do".
For example with defensive towers, can be capturable only if certain HP is reduced to specific (%).
this would apply to Fortress.
The second