square... outside the tower?
that must be a euro thing..

It is an ICAO thing, I believe. These:

http://tinyurl.com/6knnaw

are the German regulations on marking aerodromes with VFR traffic, and they
claim to be in accordance with ICAO annex 14, volume I. The landing-T is
shown on page 3.

and what if there is no tower?

There is a signal field between the operations building and the runway, even
if there is no tower.