Fit locks which secure the frames together in preference to locks which simply secure the handle or stay bar. Casement windows, by their very design, need to be locked by securing the window to the frame. Most casement locks are screwed to the window rather than the frame – a weaker system. There are locks which are fixed to the frame and these are probably more secure, as well as being easy to use.
There are also locks specifically designed for use on windows with tapered edges (not 90º to the frame).
If the window is flush to the frame fit mortice rack bolts (bolts fitted into the window and operated internally with a key). They should always be fitted at 90º to the grain of the wood, reducing the likelihood of the wood splitting if subjected to pressure and fixed into either end of the frame. You might want to consider using door mortice bolts for a longer throw.
If you are intending to use the type of lock that screws to the window, the following test is a rough guide to help you to decide how many you will require: with the window closed, press each opening corner. If there is any movement, fit a suitable lock, such as a push lock (push to lock, key to open) at each corner on the opening side. If there is no movement, a lock fitted to the centre of the opening frame will suffice.
It is possible to secure wooden casements in the open position for purposes of ventilation, very necessary in hot weather, or child safety.
Likewise they are well suited to upper floors where windows can be locked open, allowing ventilation but, at the same time, preventing children from opening the window beyond a safe aperture. Whilst it is recognised that this type of device is designed more for safety purposes rather than security, it would be likely to alert the occupant if an attempt to force it was made. It is intended that these locks should be used only when someone is in residence. If your home is unoccupied the windows should be locked in the usual manner.
If you are considering replacing your existing windows, look for the new British Standard 7950 kite mark. It has been established to set specific manufacturing standards of design and security for casement windows.
Note: Georgian wired glass may look strong, but it will easily break. It is a fire safety glass not a security glass.