You will see a countdown timer in the left section of the game window when someone is planning to attack or invade your Sector/Castle/Haven. Go to the Command Center/Keep/Pirate Stornghold and open the "Tracking" tab to find out who's launched an attack against you. Open the "Send" tab at Command Center, activate the "Enemies" filter, and choose the attacker's Sector coordinates to get to their Sector. You will return to the Map next to your enemy's location when you close the Command CenterKeep/Pirate Stornghold window. After you check the Attack Timer, use the Mail Network or any other method to call for Reinforcements from your Friends.