There is a master difficulty setting in the Preferences Screen where you can adjust the level of difficulty up or down as needed. Along with making opponents weaker or stronger, it also changes some other subtle parameters to make the game more suitable to taste.
Even at Moderate Difficulty the game can be quite challenging; Hollow Ground was designed with hardcore gamers in mind. If you are having trouble, turn the difficulty down to begin with and hone your skills until you are ready for the tougher settings. Once you get a firm grip of the game's tactical side, you will notice that there is a great deal of re-play value. Re-playing does not become repetitive once you understand where the true variation in the game comes from. Although Hollow Ground has some similarities with first-person action shooters, it is more a game of tactics. Hollow Ground will exercise both your trigger finger and your brain.
There is also a hidden difficulty mode called Suicide that is activated after you have finished the game once. Suicide difficulty is not impossible, but it is so hard that you need to be a true Hollow Ground master to beat the game at that insane difficulty.
Notice: If you are a very impatient person, you might want to cheat by holding down Alt while selecting the general difficulty setting in the Preferences Screen to enable Suicide mode without actually having beaten the game. On the other hand, if you can't beat the game, you will not survive more than a few seconds in Suicide Mode. This can of course also be handy in case your high score file is lost or deleted.