The Fort is necessary for protection from monsters and from sieging Clans, who are trying to loot their Fort in a Clan War. The Tank is used to attack other Clans' Forts in a Clan War.
Forts can be fortified with a variety of defences, such as sniper towers, machine gun turrets, and ground-based traps, with the aim of eliminating attacking forces before they can destroy the base.
Tanks can be customized and upgraded to increase speed or damage output.
Players can also decorate the Fort and Tank. It adds some aesthetic value to the game, allowing players to enjoy their surroundings as they engage in battles. These decorations have no apparent functional significance.
Both your Fort and Tank need more parts to gain access to new items and for upgrading.
Opening the Engineer Chest will allow you to obtain new parts for the Tank and Fort.
Once you have enough parts to upgrade the item, the progress bar will turn to "Upgrade". After you start the upgrade, any Clan member can speed the build time up with gems.
Every item upgrade adds to the total level of your Fort/Tank. On the top left corner of the screen is a progress bar indicating your current Fort/Tank level and the number of additional points required to reach the next level. As your level increases, you'll have more energy to use for placing items.