The portico forms the central passageway and is a clear nod to the porta pia, michelangelo's famous city gate in rome. It is an ode to the architecture of antiquity. Can you see the two 'bucrania', or ox skulls, on the left and right sides in the upper corners? They are a reference to the 12 labours of hercules. Similarly, the rams' heads represent patience, or christ's sacrifice. Or was rubens' primary intention to represent prosperity? He was a hard worker, so that could well be the case.
The mythical satyrs, forest creatures that are half man and half beast, symbolise how you walk through the portico straight into nature. The dolphins represent eroticism and love. The portico is an ingenious work by rubens: brimming with symbolism and craftsmanship.