The Clivus Capitolinus is a road/street. A Clivus is just a Via (road/street) like in the Via Sacra (Sacred Way or Sacred Road/Street) except Clivus means a steep road/street. In this case it is a road that climbs up the Capitoline Hill, so basically a steep road (Clivus) to the Capitoline Hill (Capitolinus).
This road starts over in the SW corner of the Roman Forum Square, so if you are still sitting behind the Rostra facing the Capitoline Hill, 90° to your right is the Arch of Septimius Severus and 90° to your left is the SW corner of the Forum Square. It's where the 90° turn in the road is on the left side of the Temple of Saturn. This road runs alongside the left side of the Temple of Saturn, then turns and cuts across the front of the Temple, then runs uphill along the right side of the Temple, past the Portico of the Dei Consentes and up to the Capitoline Hill.
Originally this road was just a simple dirt path but at the end of the Regal Period (509 BC) it was made into a road (unpaved) now suitable for wheeled traffic.
The original dirt path and later road led to the Arx which was the low point between the two higher peaks of the Capitoline Hill O----O. The Arx is roughly where the modern Piazza del Campidoglio is today, so this original path/road turned into the Hill just past the Portico of the Dei Consentes and went to the saddle between the two peaks. Also a branch was added to the Temple of Jupiter on the left or South peak of the Hill when this dirt path was turned into a road. This path was the only approach to the Hill until the stairs (mentioned earlier behind the Arch of Septimius Severus) were built to the Arx.
It was first paved in Lava stones in 174 BC and a porticus was erected on the right side of the road from the Temple of Saturn to the Temple of Jupiter (or very nearby). Sections of this original 174 BC paving still exist (right side of Temple) along with sections from Sulla's rule 80's BC (probably left side of Temple). The section in front of the Temple/Saturn is one of the best specimens of Augustan Era (31 BC-14 AD) paving in Rome.
Ok let's start at the beginning of this road which is on the left side of the Temple of Saturn or SW corner of the Forum. Sulla (General, Consul, Dictator) in the 80's BC rebuilt this section of road but most importantly the ground below the road. A series of small underground arches running the length of that section were built to raise up that section of road ^--^--^--^ which was then paved over. Perhaps the surrounding area's ground level had risen over the centuries and these arches raised the road to the current ground level? Or perhaps it was to make the incline more gradual and less steep? As you can see the road that runs alongside the Forum Square also inclines up at the end to meet this Clivus. Remember this road carries wheeled traffic also.
Then the road U-turns in front of the Temple of Saturn and starts up the Hill. This uphill section is the most interesting. Now originally it turned just after the Portico of the Dei Consentes and I mentioned in that section that the back walls of those rooms on the left behind the colonnade was actually a supporting structure for this road. But our road today goes straight and doesn't turn at the Porticus. This I assume either was the original branch-off (doubtful) to the Temple of Jupiter but more than likely a much later branch.
The Clivus becomes an important road when the Romans start doing Triumphal Parades which end at the Temple of Jupiter, so the section that goes straight to the Temple becomes the main street. So now we have a main ceremonial street with a porticus (covered sidewalk) along the right side.
In 190 BC Scipio Africanus erects a nice decorative arch at the top of the Clivus. Livy says there were nine gilded bronze statues and a pair of horses on top of the Arch. The statues were very likely of Scipio and his family. Also Cicero mentions private homes along the Clivus, both the arch and houses were probably in the Arx section of the Clivus.
Halfway up the Clivus close to the Temple of Saturn very likely on the left was the Porta Stercoraria. This was a closed gate that opened into an alley (Angiportus). Every June 15 the gate was opened for a 'taking out the trash ceremony' :-) . In the Temple of Vesta the Vestal Virgins kept a sacred fire burning 24/7/365. The ashes from this sacred fire are stored beneath this Temple. And once a year these sacred ashes (stercus) were removed and taken through this gate and over to the Tiber River and dumped in.
A 'Triumph' or 'Triumphal Parade/March' was awarded to a victorious General or sometimes the Emperor who was just grabbing all the glory of his General's victory. The Clivus was the last leg of these military parades. The parade would enter the Forum with the General or Emperor in a ceremonial chariot followed by wagon loads of war trophies (gold/silver/gems, money, artworks, anything of value), captured chained leaders (Kings, Rulers, Generals) to be publicly executed and POW's and civilians to either be sold into slavery or killed in the Games for entertainment. The captured leaders, POW's and civilians would branch off somewhere in the Forum away from the parade and exit by the Curia. But the victorious General or Emperor with his entourage would continue on to the Clivus Capitolinus and up to the Temple of Jupiter. There the General or Emperor would sacrifice a snow-white bull to Jupiter to thank and honor him for their victory.
Take Julius Caesar's Triumph where the signs 'veni, vidi, vici' (I came , I saw, I conquered) were carried, it lasted three days. And later Brutus and his fellow conspirators marched up this Clivus right after they murdered Caesar and barricaded themselves in the Temple of Jupiter.
Also troops would be stationed here when something was going on in the Forum (speeches, funerals, ceremonies, etc) in case the locals got rowdy. Remember the 'Year of the Four Emperors' when the future Emperor Domitian is on the Capitoline Hill under attack and later escapes dressed as a priest after the battle? This is from 'Tacitus' who details the battle fought on the Clivus Capitolinus:
"Martialis had scarcely regained the Capitol when the infuriated troops appeared. They had no leader, and each man followed his own devices. At a rapid pace, the column galloped past the Forum and the Temples abutting on it, and charged up the slope opposite as far as the outer gate of the Capitoline Hill. At that time, there was a row of *porticoes on the right-hand side of the Clivus Capitolinus* as you go up. The defenders got on to the roof of the colonnade and assailed the Vitellians with stones and tiles. The enemy for their part were armed only with swords, and thought it would take too long to bring up artillery or missiles. So they hurled firebrands at a projecting portico, followed the flames as they spread uphill, and would have forced the charred gates of the Capitol, had not Sabinus uprooted the statues with which past generations had adorned the whole area, and so formed an improvised barricade at the actual entrance".
A bit higher than the Portico of the Dei Consentes on the Clivus Capitolinus, you might see the paving stone in the middle of this photo, into which a circle game board has been carved.Next: #13: Temple of Saturn