> For the complete documentation index, see [llms.txt](https://castleoverlord.gitbook.io/whitepaper/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://castleoverlord.gitbook.io/whitepaper/getting-stronger/units-upgrade.md). # Units Upgrade A unit in Castle Overlord will gain experience points by participating in battles. After reaching a certain amount of experience points, the player can upgrade that unit by spending gold and materials. Stronger units will help the player win harder battles, and will also be more desirable in the NFT marketplace. Upgrading units is not mandatory, but owning strong units is highly recommended in order to progress in the game. If they wish to, players may choose to recruit more units instead. Winning by quality or quantity, the choice is in the players’ hands. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://castleoverlord.gitbook.io/whitepaper/getting-stronger/units-upgrade.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.