Hi.
We (8 people) have spent really nice time in Castelinho São Marcos. Location is perfect for someone who wants to do sightseeing (for beach lovers I would suggest other place). few minutes to ferry to Valetta, nice restaurants in neighbourhood, quiet street. Well prepared information for guest. In my opinion perfect for max 8 people (2 per room). If you are coming from "flat" country - be prapared for stairs everywhere :-) (including your vacation house :-) ). There could be more space in 2 rooms (less beds) but maybe someone needs many many beds. Owner is helpful - reacts quickly and immediately answers your questions.
Note that in room 1 bathroom is one floor down and outside window is only 40x40 cm - the other window shows inner part of house.
Go to Malta - it is wonderful (even if it is 40 deg Celsius ;-) ).
Tomasz