When visiting the traditional fair in San Telmo, which mostly goes along «Defensa», try to go to The «paseo San Lorenzo» which intersects with Defensa at number 755. It is a tiny street and has the oldest still existing house of Buenos Aires. You will notice the murals and the odd layout of windows and doors. The top of the current door used to be the ground level, when, one block down the road was still Rio de La Plata and the City of Buenos Aires a lot smaller. There where older houses, but they have been destroyed eventually...
If you want to know more, ask the old painter, selling his paintings in that street. He is a good source of stories about San Telmo. But, do NOT go in the evening or generally in the dark. That is when drug-addicts, prostitutes and thieves outnumber more trustworthy people... ;-)