1. Introduction of the game:
Word Stacks is a captivating word puzzle game that challenges players to shift, shuffle, and combine letters to form words and clear levels. With its serene backgrounds and soothing soundtrack, the game offers a tranquil yet stimulating environment for word enthusiasts.

2. All characters and levels of the game:
While Word Stacks is not character-driven, it boasts an extensive collection of levels that gradually increase in difficulty. Players progress through a vast array of stages, each presenting a unique set of letter stacks and hidden words that cater to a wide range of vocabulary skills.

3. What attracts users most to the game:
Users are most attracted to Word Stacks for its innovative gameplay that blends classic word search mechanics with clever puzzle elements. The game's minimalist, clean design and the satisfaction of discovering hidden words make it an enjoyable experience for players of all ages.

4. Why it is popular:
Word Stacks has gained popularity due to its ability to provide a mental workout in an enjoyable format. It's accessible for quick sessions but also provides depth for those seeking a more extended challenge. The game's popularity is also bolstered by its daily challenges and the sense of achievement from mastering each level.

5. The most difficult part of the game:
As players advance, Word Stacks introduces increasingly complex puzzles, with the most difficult part being the levels where numerous word solutions are possible. The challenge lies in identifying the precise combination required to progress, which can be a true test of one's vocabulary and critical thinking skills.

6. Strengths and weaknesses of the game:
Strengths of Word Stacks include its user-friendly interface, the progressive difficulty curve, and the ability to stimulate cognitive skills. The game's weakness may include a potential plateau in variety for advanced players and a reliance on in-app purchases for hints which may deter some users from continuous play.

7. What other games have been developed by the same developer:
PeopleFun, Inc. has developed several other popular word games, such as Wordscapes, Word Chums, and Wordscapes In Bloom. Each game offers a unique twist on traditional word puzzles, retaining the core appeal of Word Stacks while providing fresh gaming experiences.