Can I react with guards hidden at a battlefield after Downwell resolves?

The answer to your question depends on whether any other triggered abilities were generated by Downwell's resolution.

Based on the rules and RiftJudge FAQ #10642, here is the breakdown of your situation:

The Short Answer

You cannot react with a hidden card immediately after Downwell resolves if Downwell was the only thing on the chain. If the chain becomes empty after Downwell resolves, the game state returns to "Open." Because hidden cards are discarded during the cleanup step if the battlefield becomes uncontrolled (per RiftJudge FAQ #8494), you will lose your hidden Guards! before you have an opportunity to play them.

However, if Downwell's resolution triggers another effect (like a "When this leaves the board" trigger), that trigger is placed on the chain as a pending item, keeping the state "Closed." You can then use your hidden card to react to that triggered ability.

The Sequence

If Downwell resolves with no other triggers:

1. Downwell resolves, returning all units and gear to hands.
2. Downwell finishes resolving. The chain is now empty (the game is in an Open State).
3. Cleanup occurs: Because there are no units at the battlefield, it becomes uncontrolled. Your hidden Guards! are moved to your trash (Rule 323.7 and RiftJudge FAQ #8494).

If Downwell resolves and generates an additional trigger:

1. Downwell resolves, returning all units and gear to hands.
2. A "When this leaves the board" trigger is placed on the chain as a pending item.
3. Because an item is on the chain, the state is Closed (Rule 187.4.c).
4. You can now play your hidden Guards! as a [Reaction] to that triggered ability.

Important Constraints

• Targeting: Per Rule 811.1.d.3 and RiftJudge FAQ #9757, because you are playing Guards! from a hidden position, the Sand Soldier token must be played at the same battlefield where the Guards! card was hidden.
• Timing: You cannot play a card from "Hidden" on the same turn you hid it (RiftJudge FAQ #9904).
• Standard Moves: You cannot react to the loss of a unit if it was moved by a "Standard Move," as that does not use the chain and does not create a reaction window.