I was in the Canary Islands recently and during my stay several large, round, flying insects flew into my apartment through the open window.

These can best be described as approximately 2 inches in diameter, dark coloured, and very hard. We tried to squash a couple of them and it was almost like they were armour plated! I thought a t first they could be flying cockroaches, but they were ROUND in shape.

Unfortunately I didn't get the change to take a pic. Any idea what they could have been?

Sounds like a beetle of some sort: hard-shelled, flying, large. Unfortunately I cannot find much on Beetles of the Canaries in English, although several sites say there could be as many as 4000 different species there.