So, first of all let’s take a look at what type of windows you have? Do they open inwards, outwards or are they sliding windows? If they open outwards, you will not be able to fit fixed window grilles if you would still like to be able to use your windows.
In this case sliding grilles would be a better choice for you. If your windows slide open you should be fine depending on the design. If your windows open inwards once again this should also be fine.
If you send us some pictures or videos of your windows, we will be able to advise you over the phone.