My flat got invaded by ants this week - really tiny little buggers. I work from home most of the time so I was sitting at the table in my kitchen/living area working on the laptop and all of a sudden there were a few of them on the top of the table. I squashed them of course... but then there were more... after a while I tried to figure out where they came from and I still do not understand where they came from.
I live on 3rd/4th floor so it is not like they could come in from the garden, Could have been from the roof terrace but I actually do not think so. They seemed to come from a corner of my flat at the opposite end - almost as if they are coming from below the floor. Or even from the abandoned place next door.
I have now sprayed all possible places they could come from and the flat is ant free (for the moment at least). They better stay away or else... 
Oregano
Pro 
We had a major invasion attempt in our house about 3 weeks ago. There were columns of ants going up the kitchen wall and disappearing in a tiny crack in the ceiling.
They can come through tiny cracks and open windows or doors. We put down ant powder across entrances and places we had seen them and have seen only the odd one since then.
O.