Have an alarm system installed that includes motion detectors in your basement.

Realistically, though, I'd be less concerned about windows that face onto a heavily used sidewalk...and more concerned about the windows that are out of easy sight....

Thank you for not wanting to go the route of metal grilles!

The basement floor will soon be dropped a couple of feet to make a nice new guest and tv room. Security is important. A motion sensor would be a pain. I hate to put in the metal grills but might have to unless there are other ideas. Thanks for your help.

You could install either security film or laminated glass.

Here is a link that might be useful: Security film

Victorians did sometimes have decorative grilles, the other possibility might be shutters....so a retro-styled grille could be one option. You must ensure there's a way to open them if you need to depend on them as a fire exit.

The big thing about the grilles are they are a visual deterrent. As for securing a sash window, of course the most effective way is a bolt that goes through the bottom sash to the top (assuming that's what you have)

A visible alarm system is a good deterrent. There are numerous wired or wireless ways to secure the window as well, plus, you could have glass-break detectors facing the vulnerable windows - that will give you the best early warning.

The downside always is, without a visible (ie ugly) deterrent, the would-be burglar does not discover the premises are secure until they've already broken something.

Given all the options, you might want to consider "security film" which is a plastic window film applied to the glass (can't remember if it goes inside or out) - you can have tinted, or non-tinted (which has UV protection as an added benefit) - it was developed I think for the UK during the Northern Ireland troubles, to stop windows from shattering during bomb blasts. The glass will still break, like a windshield, but it will stick together. Much harder to get in.

Lastly, you could reglaze the windows with Lexan, some of it offers good thermal insulation qualities too. That would make them more or less 'unbreakable' to some extent but of course they can be scratched etc but Lexan's a great plastic with excellent clarity.

All helpful suggestions. The windows are that old rippled glass and I hope to keep it but I might add the window film on the inside. Noise might be an issue so its possible I might have to reglaze w/ laminated. The windows don't open; I think I'll need to look into the fire code and find out what egress requirements there are here. Thanks again for the feedback. Much appreciated.