They built a little hill so spotters can overlook the runway and fences from a higher position. There‘s also an old MI-8 next to it that was converted to a snack bar, it‘s pretty sweet to hang out for the day.

Whatever happened to observation decks open to the public? Tegel has one, and I remember one in Frankfurt

Looking at this they seem somewhat of a german specialty, but now with entry fees at bigger locations.

https://eu.usatoday.com/story/travel/columnist/hobica/2013/1...

RDU (between Raleigh and Durham, NC) has one: https://www.rdu.com/observation-park/

also conveniently the best spot to wait to pick up an arrival (as they like to keep traffic flowing outside the terminal)

Manchester airport has a viewing park along with a Concorde in a hangar you can walk around. (I’ll take any opportunity to plug a visit to Manchester)

Yeah, I don't know. Krakow had one huge observation deck, now it's all gone and there isn't a single area of the airport which would have the view of the runway(not without going through security first).

I think BWI has one if you're in the right spot. It was closed the last time I was there, however.

Yeah, there's one at DUS, with security control and indeed a fee.

Yeah in Vienna, Austria we also got a little hill (and also those holes for ages)