After the intro you'll find yourself in prison blocks much like those of Duke Nukem II. Grab the 
spheres (they're for scoring purposes) and then shoot the container to release the slime monster inside. The canister only took one shot but the slime takes two. Some of the slimes are able to break out of their canisters of their own accord and once out they'll bounce around the ceilings and generally be a nuisance. They can also grab onto you and kill you if you don't shake them free. Basically, you should always frag them the moment you can to avoid these problems.
Continue to the right. The forcefield does three short bursts and then has a gap, so wait until it's off then dash through. After this are some weird robots that takes four hits each, as well as some more spheres and slime containers. The robots do stop and shoot now and then, but this should be easy to avoid. Watch out when destroying them, though: their base part will explode, and you'll be hurt if you're caught in the explosion.
After this you'll come to two more forcefields. The slime between them is able to free itself, so be careful when crossing the forcefields. The
small crosses heal some damage if you've taken any.
Up next you'll spot an 
. You need to find the five letters (
) in each level in order to clear it. Now climb the ladder (
). You can shoot while climbing by holding 
or 
and pushing 
. 
makes you drop off the ladder. Note that if you fall a long way you'll take damage.
Once at the top, jump across and immediately start laying into the slime and the robot. Up next you'll find the . Ignore the ladder for now and continue left. Grab the gem and head up the ladder, then jump over to the right and head right, staying on the path. Grab the 
crystal. Now head back left and jump towards the platform. You can grab the edge by holding . When it's a good moment press to climb up and then lay into them. Grab the 
—this temporarily doubles the damage you deal—and the 
red keycard. Head back to the ladder.
Drop down onto the ladder by falling onto it and pushing . Now edge yourself up so you can shoot the robot on the left without being hit by it. Jump over to the left and grab the minigun. This one has limited ammo, but you lose it at the end of the level so you might as well enjoy it. Further to the left you'll find a 
big cross: this almost fully heals you. Shortly after that you'll find the . Continue left and cross the gap, ignoring the ladder. Deal with the slimes then mantle onto the upper level. The blue container is ammo for the gun you picked up before. Now go back and down the hole. Head right, grab the two gems, then go back left.
Push on the keycard slot to disable the forcefield. Grab the . Step on the platform and use and 
to control it. Grab the floating candle things and head right. Grab the and climb the ladder. You now see a score readout, and, depending on your success, you may gain some extra lives. Be sure to save before you continue.