Bricks

Bricks are a currency originating in Scrap Clicker 2 and were later introduced in ScrapTD. In Scrap Clicker 2 they are used to buy some upgrades while in ScrapTD they are used for buying and upgrading barrels and defenses.

In Scrap Clicker 2
Unlocked at x15 Stars, Bricks are passively accumulated. Starting at 1 per second, production can be permanently and repeatedly upgraded in the following ways with all multipliers stacking:
 * Upgrading overall Star multiplier: x4 multiplier
 * Spending levels: multiplier *
 * Upgrading "More Bricks" upgrade by spending Bricks: x1.25 multiplier
 * (40+ Stars) Purchasing specific upgrades in the Skill Tree: multiplier
 * (115+ Stars) Upgrading the scrapyard v4: x1.5 multiplier/level

* Levels required increase by 1 every time production is increased by this method, however this can be lowered by purchasing the appropriate upgrades in the Skill Tree.

Upgrades
More Steel Beams' cost multiplies by 8 with each level, all other upgrade's cost is x4 per level.

At the start it states that the max level for each upgrade is 100, although when you reach the "limit", it automatically upgrades the max level to the aprropiate number, unless stated otherwise.

Brick Multipliers
Once the player has achieved the top Golden Scrap Multiplier, they can begin earning Brick Multipliers, which multiply their brick production (if they are above the Barrel 103):

In ScrapTD
Bricks can be used in ScrapTD to unlock Defenses and Barrels and level them up. They are awarded after every match if the player has energy left.

Arena Brick Multipliers
You can get more Bricks after every Match by getting into a higher Arena. Apart from having more Scrap in a Match, this is currently the only way to get more Bricks. Ziegelsteine