- To make flags, just use a "flag" entity. Use the "teamnumber" key to specify the team. A teamnumber of 0 will be a white flag. For the capable area, you need to create a brush then convert it to a "base" entity. Also use the "teamnumber" key for that.
- Brushes can't be concave, so you have to make a wall with windows out of multiple brushes.
- For siege, put base entities outside of the base. Defenders who touch it will be killed. Offense will have to take the flag to the base entities.
- Custom textures are kind of complicated. A tutorial is probably in order for those.
Note that with base brush entities, they can only be rectangular, not at angles or in odd shapes. Just the min/max of the brush entity is used. See the docs page for more info on entities. It's a bit outdated, but it should help.