Terraria (Wiki): Reef Platform

The Reef platform is a solid surface designed for walking and traversing.

The Reef platform is versatile, serving multiple purposes like bridging gaps without causing destruction, granting rooftop access while blocking certain enemies, or creating an elevated combat zone for players to shoot down at enemies while remaining untouchable.

The players can stand, walk, or jump horizontally on the Reef platform, which functions similarly to regular blocks. Players can press the Down key while standing on the platform to descend. However, these platforms are stable when landed on from above.

The Reef platform consists of four unique placements. Players can alter their position using a hammer, similar to modifying a standard block:

• A first hammer strike will transform the structure into slanted stairs facing right. Additional visual supports beneath the slanted part may occasionally be present, depending on nearby blocks, but these are purely for visual purposes.
• A second hammer blow will cause the structure to switch to stairs facing left. 
• A third hammer blow moves the object to its "bottom" position, making it horizontal and lowered to the bottom half of the block. This allows for the attachment of banners and other hanging items beneath it. However, chandeliers or lanterns cannot be attached to this position (3DS and old-gen consoles).
• A fourth hammer strike resets the platform to its original position. Note that only one platform can be on a block tile at a time, so the top and bottom platforms cannot be on the same tile simultaneously.

When the object is in its default or "top" position, it occupies the upper half of the block space and allows furniture placement, such as Bottle, which becomes a Placed Bottle for potion making.

The Reef platform is not lava-resistant compared to other platforms in the game; they will disintegrate if dropped, intentionally thrown into it, or upon contact. The Reef platform supports gravity-affected blocks, such as Sand. Players can eliminate Reef platforms in a row with no negative effect on movement, just like regular blocks. Since the player occupies two tiles horizontally, they will not fall through gaps.

Staircases are versatile in orientation, regardless of the attached structure. The players ascend when facing the same direction as the stairs they climb, while stairs facing away from them have no impact. Players can bypass stairs by pressing the Down key, and the Smart Cursor feature allows for convenient positioning on platforms without needing a hammer. Stairs are vital for various advanced movements strategies like hoiks, impassable doors for NPCs, elevators, and enemy-catching traps.

The Reef platform is capable of having hooks attached to it. While holding a platform (3DS, old-gen consoles), players cannot grab any other object, regardless of the grappling hook's direction. The grappling hook will ignore any Reef platforms below the player's body (desktop, mobile, console). 

Ropes can now be integrated with the Reef platform, and players will fall through platforms while riding a Minecart. Piggy Banks, Chests, crafting stations, and other types of furniture can be placed on Reef platforms just like regular blocks. However, there are a few exceptions, such as Gems and Reef Shards. The Reef platforms can be used as shelves to store furniture in a limited space without limiting the player's movement.

Walking enemies can walk or pass through Reef platforms, while flying enemies like bats can fly upwards but not downwards through these platforms.

On the other hand, certain enemies like Harpies completely disregard platforms. Non-playable characters (NPCs), however, will always climb up platforms. Wraiths cannot ascend using platforms, but they can remain suspended above them.

Projectiles and weapons can pass through the Reef platforms.