Castra Regina was established under the Emperor Marc Aurel in 179 AD, as a military camp for 6000 men with its strategic location being the northernmost point on the Danube. This fort was approximately 450 x 550 meters large with a mote surrounding it 6 meters wide and 3 meters deep. The fort’s wall was 1.8 meters thick and 9 meters high, with 30 towers and 4 entrance gates. Today, portions of the wall can still be seen at a few points including porta praetoria, the parking garage on Dachauplatz, Am Königshof, and Ernst-Reuter- Platz. The north gate of the fort, once called Porta Aquarum (water gate), was renamed after the completion of the street ‘Via Praetoria’ and thus taken its preset name, Porta Praetoria. The area was unearthed in 1890 during the construction of the Bischöfl ichen Brauhauses (bishops brewery), after being covered in the late middle-ages. Archeologists believe the gate had double entry ways with two arches giving access from the east and west. A tower on both sides whereby each had 2 fl oors with 5 roundedarched windows to defend against attacks. Above their 4 meter wide doors was also another windowed fl oor. Much of the Porta Preatoria gate is still visible today. It is made from 13 limestones which were rounded and chipped to fi t against each other to exact precision. die Porta Prae- Along with the larger Porta Nigra in Trier (180 AD), the Porta Praetoria is the oldest above ground structure north of the alps.