The enemies of the BATTLEFIELD and indeed many other places react more aggressively towards you if you have a low FORFEND score. You can increase your FORFEND by making and equipping various pieces of equipment at HOME.
The purple cross is a LORDSTOP which is held for 8 seconds and despawns one LORD upon contact.
(Edit: Something else I forgot, when you PULSE TOOLCUBE, it slows down LORDS hugely for a couple of seconds. This lets you control their placement and get around them without using up the LORDSTOPS too soon.)
There are two "huts" in the BATTLEFIELD which act as RESPAWN points when you enter them.
The "candle" forms a plant and root bridge shortcut from the first hut if you spend 100 AGE to make it grow.
There's a bit over 2000 AGE in the BATTLEFIELD which can be used to finance early game activities, but it is not required that you go to the BATTLEFIELD as soon as the red door is visible at HOME, and is more likely to end up an early grave if you are not equipped sufficiently to dissuade the enemies from acting at maximum aggression.