In a new document, insert new layer folders so you have 6 of them.
Make your canvas 600x600.
In "Layer 1" folder, draw three rectangles, two for the towers and one for the main wall.
Draw a smaller rectangle for the third tower, a bit taller than the others.
Using the Rectangle tool, add some posts to the wall.
Rename "Layer 1" as "Walls" (double-click on the layer name).
Rename "Layer 2" as "Roofs". In this layer folder, draw triangles for the tops of the towers. To draw the triangle, you have 3 options:
Make sure the width of the bottom matches the width of the rectangle.
Rename "Layer 3" to "Flags".
Draw a tower flag. First draw a flag pole using the Line tool.
FLAG: draw the flag using either
Zoom in and adjust the points to resemble a flag using the white arrow tool.
There are 2 ways to draw the gate:
Option #1 (easiest): draw a rectangle, then ungroup it (Modify>Ungroup or CTRL+SHIFT+G). With the Pen tool, click and drag a little on the top corner points. This is much easier/simpler.
Option #2 (trickier): a rounded rectangle and straightening the bottom parts (using ALT and the white arrow tool). This can be tricky to get the bottom lined up though.
Add some lines to the gate for the wood. Go to the fill properties, pick a grey or brown color, and add the Texture "vertical lines" to the gate.
Rename "Layer 6" as "Bricks".
Add some bricks to the walls using the Rectangle tool. Create a group of 3 bricks.
Fill them now (or later) with a color/texture/gradient.
Select them and copy them for the other places so they're all the same size.
In a new layer folder, add 2 rectangles - fill with colors, then choose a Texture you like. Make sure they don't have a stroke. Move them behind all the other layer folders.
More examples...