When both your HQ and your Defensive Line reach Lv.30, you can rebuild some Buildings.

Only the following Buildings can be rebuilt: HQ, Air Force Factory, Tank Camp, Commando Camp, Copter Camp and Artillery Camp.

Tips: HQ Rebuild Lv.1 is required before you can rebuild other Buildings.

 

You need to complete Construction ×10 to unlock Rebuild feature. After 10 constructions, you can rebuild the building to reach Rebuild level 1. Every time you complete the Construction, you will gain bonus.

When Construction in the progress bar reaches 10, you can start Rebuild on current building. Each Buildings can be rebuilt 3 times.

Each Rebuild provides additional Production feature and Rebuild privilege for the Buildings. Rebuilding the four training bases will unlock 4 new troop units. The privileges you can get through Rebuild can be viewed by tapping the Buildings > Details > More Info > bottom of the interface.

 

For example, HQ Rebuild privileges: