That was my initial reaction when I saw this doorway just standing by itself, that it reminded me of the Stargate from the TV show (yes I know it isn't round). It does seem a little strange just standing there by itself near St. Cajetan Church, but it is interesting and you should be sure to read the plaque about it.

Once you saunter into the shadowy covers of the compound of the picturesque St. Cajetan Church in Old Goa, you might fail to notice on your left a gateway of basalt pillars holding up a horizontal lintel. It took me some time to figure out the location.
This is all that remains of a palace constructed by Yusuf Adil Shah, the sultan of Bijapur.The palace, prior to destruction, was the residence of the Portuguese Governor until 1696. It was demolished in 1820 for construction of new houses. This is the only remnant of the Adilshahi rule in Old Goa that is now dotted by a plethora of Churches.
The palace must have seen its days of glory. But now all that you see is the gateway and an informative plaque. Strictly for history aficionados.

The historic palace of Adil Shah, also known as Palacio de Idalcao by the Portuguese, now houses government offices. The original structure was destroyed by the Portuguese and then rebuilt in colonial style. This building can only be viewed from outside