No, that one doesn't say she arrived at the tower--just that she swam that far. A famous example of this difference between English and Spanish is "He walked into the cave" which must be "Entró en la cueva caminando" not "Caminó en la cueva," which would mean he was already in the cave and did some walking there. [edited to fix the error snowdove pointed out.]
There is the hasta. For Ella nadó en la torre., I would agree. But hasta gives the information of movement from somewhere to the tower, not into it, just that far.
Which I thought was the meaning of myperu's sentence. If not, sorry.
I thought that She swam into the tower. would have been the sentence to say that she get that far AND entered.
I agree completely and have been getting this one wrong repeatedly over the years. In English I cannot find or think of any difference between the sentences "she arrived to the tower swimming" and "she arrived at the tower swimming" at all. However she is getting there is not important and I realize that both of the above sentences might be somewhat differently translated from English to Spanish they mean exactly the same in English and should both be acceptable. Arriving at a tower could be outside of the tower and arriving to the tower would be the same. This question was asking for an English answer, so both should be correct.
"to arrive" is a different matter of course. "arrive at" is correct and "arrive to" is incorrect. The logic behind this (if you can call it that) is that "to" is a transitional preposition. "arrive" isn't about transitioning, it describes the end point of the transition, so "at" (or sometimes "in") is the appropriate choice because it is used for location.
I've read that Spanish treat gerunds as adverbs whereas English treats them as nouns. This could be a good example of that. The sentence doesn't work in English because 'swimming' is being positioned as a noun instead of an adverb.
I'm not sure any version of this sentence that maintains 'arrive' would ever sound natural in English, probably the closest is She arrived at the tower by swimming. It's more likely we would say She swam to the tower or She was swimming to the tower.
Actually the Spanish Gerundio (the equivalent of the English Present Participle--a word ending in -ing) has only two uses!
The Spanish Gerundio (a Spanish verb ending in -ando or -iendo etc) can only be used to: 1-- Form One of Progressive Tenses or 2--As an Adverb modifying a Verb
In this particular sentence "nadando" is modifying the verb "llegó"!
She arrived swimming!
The English Gerund on the other hand exclusively means a Present Participle acting as a noun!
Again English and Spanish have different meanings for the cognates!
There's nothing wrong with the sentence in Spanish or English. It's just another one of those curveballs that Duo likes to throw from time to time, where you have to think it through and (as a few others have said) trust your understanding of the language rather than what you think it should really say.
This sentence means that when she arrived at the tower, she was swimming. Other similar constructions could be 'she arrived at hospital breathing', 'at the summit sweating', '...at the museum talking', '... at the finish line running'. That last example might be the key to understanding Duo's original sentence.
Is the 'place' essential in this Spanish construction? (In English I have heard similar sentences where some phrase is used indicating 'a period of time' over which the action was continuous: Example 'She spent the morning sleeping'. I am not trying to make one language more like another but to try to spot where there are big differences that I need to remember!