Spaniard here: I can confirm to @JakeRS that having iron bars in the windows (“rejas”) is part of the traditional architecture (especially towards the south). Relic of the times when you had no air conditioning and wanted to have windows open for fresh air circulation at night (the window panes open inwards) without compromising safety (both against possible intruders and against children or animals falling off them).
In fact there are traditional songs and tales of young lovers, where he would go to his beloved’s house, he would stay outside, she would stay inside, and they would talk through the open window across the “reja” (so, no touching!). Very traditional folklore stuff, that!
As to property crime, I must agree with what others have said. Although there are pickpockets in touristy areas, house robberies, house invasions and generally robbery with violence is rather unusual.
In my experience, when there are house burglaries, they are usually targeted towards houses where well-to-do people live and they are precise: the thieves tend to know what to go for and where, and tend not to bother with places that won’t give them a very hefty profit.
Also, the building boom of the early 2000s left (after its collapse in the wake of the 2008 subprime crisis) lots and lots and LOTS of empty buildings, some of which were taken over by squatters, while others have fallen into disrepair. I do not know if those squatters are counted as part of “crime statistics”.
Summing up: Spain is (when it comes to burglaries and home invasions) quite a safe country. When it comes to pickpockets, one has to be careful (especially in touristy areas). In my experience, they are SKILLED.
(Personal anecdote: I was pickpocketed in Barcelona. Pity that what the thieves took was my “collection” of essentially worthless Romanian Old Lei banknotes; a big wad of banknotes with a total value, in the exchange market, of roughly $2.50)