According to Roman law the Pincio (collis hortulorum), the Field of Mars and the Ianiculum are outside the boundaries of the city of Rome.

The bad news:
They are therefore not eligible as locations for the primary residences of the senators and knights.

The good news:
land can be bought around them for the normal price of Italian land (Dn 150000/unit) - if it is available. It will yield income. But in the interest of historic authenticity any property there will incur maintenance costs:

Small house: Dn 25000/year 1-3 units
Medium house: Dn 37000/year 4-6 units
Large house: Dn 56000/year more than 6 units

For the purposes of this game: the land of the field of Mars is presumed to be at the moment completely bought up by private investors.