Download the attachment, work up from there!
This challenge involved working through layers of different encodings and ciphers. While none of the ciphers were too difficult to solve (especially once you identified the hints directing towards them) the challenge made for a great overview of some of the most common ones.
We start by taking a look at the contents attached file:
R3JhbmRwYSBzZW50IG1lIG9uIGEgdHJlYXN1cmUgaHVudCB0byBnZXQgYSBsb3N0IGZsYWcgYnV0IHRoZSBtYXAgd2FzbnQgY29tcGxldGVkLiBBbGwgSSBzYXcgd2FzIHRoZSBjaXR5IG9mIFJPVCBhbmQgYSBjb2RlZCBtZXNzYWdlOiBKdW5nIGxiaCBqdmZ1IHNiZSB2ZiBuZyAuLi4tIC4uIC0tLiAuIC0uIC4gLi0uIC4gLi4uIC0uLS4gLi0gLi4uIC0gLi0uLiAuIGhmciBndXIgeHJsIFFCZ2piIG5hcSBjZWJpdnFyIGd1ciBjdWVuZnIgVFBJQ0d7TXBjSHBrc2tLYmlman0=
It appears to be Base64 encoded (as also hinted by the name of the challenge), and sure enough, running the command cat based.txt | base64 -d results in the following:
Grandpa sent me on a treasure hunt to get a lost flag but the map wasnt completed. All I saw was the city of ROT and a coded message: Jung lbh jvfu sbe vf ng ...- .. --. . -. . .-. . ... -.-. .- ... - .-.. . hfr gur xrl QBgjb naq cebivqr gur cuenfr TPICG{MpcHpkskKbifj}
Now we have part of the message decoded, but the second half remains to be solved:
The first half mentioned something about the "city of ROT," perhaps suggesting that the next part of the message may be ROT13 (or some other variation of ROT) encoded.
Running the second half through rot13.com results in:
What you wish for is at ...- .. --. . -. . .-. . ... -.-. .- ... - .-.. . use the key DOtwo and provide the phrase GCVPT{ZcpUcxfxXovsw}
It's pretty obvious that the dots and dashes are Morse Code, or at least we sure hope they are.
Using the Morse Code World Translator we get the phrase:
vigenerescastle
The phrase above directs us towards solving the final piece, GCVPT{ZcpUcxfxXovsw} as a Vigenère cipher using the key DOtwo.
Using these as inputs for a Vigenère cipher solver gives us our flag:
DOCTF{WowYoureBased}