Learn Swedish: Äventyraren fortsätter till fots. - The adventurer continues on foot.

The s in fots is a fossilized genitive ending. In older Swedish, the preposition till forced the following noun into the genitive case. While modern Swedish grammar has changed, this old rule survives in fixed expressions like till fots (on foot).

Why does Swedish use the preposition till (to) here instead of på (on)?

English says on foot, but Swedish relies on this old idiom using till. You cannot translate on foot word-for-word as på fot. It must be the fixed phrase till fots.

Is it possible to use this till + s structure for modern transport, like till bils?

No, you cannot invent new ones because these are ancient, locked phrases. While we kept till fots (on foot) and till sjöss (at sea), modern modes of transport just use the preposition med without an s-ending, such as med bil (by car) or med tåg (by train).

As a refresher, why is the definite form of äventyrare written as äventyraren and not äventyrareen?

For en-words ending in the suffix -are (often describing people or professions), the singular definite form adds just an -n rather than the standard -en. This prevents an awkward double vowel, resulting in äventyraren.