Is there anything to attract them to build a nest?

Last several day's I spotted a pair first time,..amongst about a dozen tree swallows,... I know they love to go into buildings, barns, shed's etc. I followed where they are going to, ...into my root cellar, I started to build last year, I went inside and looked if they possibly started a nest, [no nest]. Since I will be working on it again in a couple of weeks I closed the entrance, now they're still flying near and check to get in.... feel sorry for them but I had to do this. I was hoping that some years they will find a place, ...I just love their chattering.

I have a little tree house and went up today and mounted a ledge inside in the hope's they might go for it?

Not sure if this is the male but was saying to me...please open that root cellar....with that twig!