The Porta Nigra, or black gate - the original entrance to the city of Trier. Much of the surrounding city wall is gone, but this gate still stands. It was erected in the 2nd century AD, before mortar was discovered/invented. The sandstone - yellow sandstone in the case of this structure - was grooved so the stones would fit very snuggly and not move. As time went on, the stones settled quite nicely. It's rumoured you can't stick a knife in the spaces between.
This is the outside view - it's a wee bit cleaner because the inside was subject to the smoke of the ages from the city it protected.
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