Frontier Castle
While it isn't much to look at, this is what a starting castle looks like. The central blockhouse holds your residential rooms, over three floors. The ancillary rooms are located outside, up against the wall. While this doesn't, quite, match the published room structure of a castle, and probably isn't as pretty, it gives you an idea of how it will look.
I think I would build it with the blockhouse in the corner of the defensive walls, leaving more space for development and, maybe, ease later development into a keep.
A castle can house up to eight people, along with a garrison of up to twenty troops. Although it only comes with five experienced security guards.
Staffing: The castle can accommodate twenty experienced troops - OR five servants, two units of Experienced Troops and one unit of Elite Troops - but there is not enough extra space for stables, so no cavalry.
Upgrades
This example has had walkways and a parapet (guard posts) added to the walls which costs 1280gp in Borderlands, or 1920gp in the wilderness. This includes twenty extra Security Guards, although the castle can't accommodate that many extra troops. If you are in a town, these can be 'live-out' security guards, or you could build an extra bunk room, in a separate building placed against the outer wall. However, that is about the limit to expansion inside the walls.
You could fortify the defensive wall with stone for 1200gp in the Borderlands or 1800gp in the Wilderness. This doesn't increase the height of the walls however, it sheaths them in stone and includes a shallow ditch around the outside.
The outer walls of the central building can be faced with stone (fortified) for an extra 1000gp in the Borders, or 1500gp in the Wilderness. ++GM Note:(This is based on the estimated length to the outher walls, and going up three stories)++