The big squares are 15' grids. The little ones around the tents are 5' ones. Anything that looks like a mudpuddle IS. Trees are difficult terrain, but the grass is normal. Spiked defenses are hazardous. Yes, it is true that the way in/way out is not easy to spot. Investigation or Perception will help here. Can't do it until you're close enough to likely be spotted, (unless we want to run a round of sneaking about) but I'll allow those checks to be made as a bonus action, while you're busy doing other things.
Make sense? As usual, I welcome feedback and questions.