Definately one of the best 2D platformer ever.
The first thing I would like to mention is this version of SMW allows you to save at any level,so you don't have to replay multiple long/difficult levels again in succession.
Otherwise,it's close to being the same game as the SNES version.
This game was so fun to play when I played the SNES version years ago,and I've played through it multiple times on the GBA.It has the same formula that most of the 2D Super Mario games have,which is you'll move from left to right,jump from platform to platform,jump on enemies or avoid them,and avoid obsticles and try to make it to the end of the level or beat the boss.In the castle stages,you'll be moving upwards a lot too.
But what makes this game so fun is the variety and creativity it has.You can occasionally find a dinosaur named Yoshi,and Mario can ride him and use him to eat some of the enemies and spit some of them out as fireballs.There is ways to make Yoshi fly too.Some platforms move,and sometimes you will need to have very good timing when you jump from platform to platform because they'll be guarded by saw blades or fireballs.
Some places have giant pillars that make you time your run past them,because they can crush you(instant death),and there is segments you'll have to swim,and you'll have to outswim enemies.You can use the fire flower upgrade that allows Mario to shoot fireballs,and there's the new upgrade that allows Mario to wear a cape and do a spin attack that kills tough enemies with one hit and allows Mario to glide long distances(which you will need for some puzzles).There is areas you can climb and you can punch enemies off the gate you're climbing on.Sometimes you will need to ride on top of moving platforms,and falling is instant death,and there is places you can bounce across springs.
You can also search for the Star Worlds too,which will allow you to use different versions of Yoshi(which can fly or breathe any enemy out as fireballs).
For more incentive to explore the levels,you can try to find 4 hidden switches which makes blocks appear,and it will make the platforming sections much easier and make certain sections of the game much easier.But 3 out of the 4 switches are well hidden in the game's big world.You can try to find all the Yoshi coins,and if you do,there's another surprise for you.
The boss fights were charming too,and instead of just jumping on them to kill them,some of them require you to knock them into the lava(using a see saw),and some bosses can walk across the walls and celing and will try to drop on you,and everytime you successfully jump on them,they'll make the room smaller and they will move faster(making it harder for you to avoid them landing on you and when they drop from the ceiling,even if they miss you,they'll stun you temporarily because they make the ground shake and they'll run at you immediately after,and if you were stunned when they landed right next to you,they will have a good chance of hitting you because they run so fast towards the end of the battle.Some bosses shoot a string of multiple,slow moving fireballs and there's bosses that hide in pipes and there's fireballs in the room that rebound of walls and you'll need to dodge them while trying to hit the boss whenever he pops out of the pipe(and it's not as easy as it sounds as the fireballs can cut off certain angles very quickly).
And there's a long,tough final boss battle with Bowser
SPOILER BELOW...
Bowser will be in a little airship and will drop machines that will walk all over the place and kill you if they touch you,and what makes the battle hard is Bowser will fill the floor with fire sometimes(and there will only be a couple of places where you can stand without being hit by the fire)and Boswer will try to stomp on you with his airship towards the end of the battle,and the only way you can hit him is to jump on those little machine versions of what look like either him or the Koopa bosses and then you'll need to throw them in the air and make them land on Bowser's head when he flies past)And from memory,I think he will try to stomp you,while having those machines walking around everywhere towards the end of the boss battle,which makes things touch.
END OF SPOILER...
The Ghost houses can be tricky to find your way out of and some ghosts will chase you if you're not facing them and there will be ghosts on multiple sides of you.The forest of illusion is a tricky place to try to find your way through too.
Other creative features about the game include the bullet bills(giant bullets that are shot across the screen from multiple directions)that you need to either avoid or you can jump of them to stop them(as long as Mario's feet land on top of them).You can use shells as projectile weapons.Some levels have screens that continously keep scrolling and if you get jammed between an object and the edge of the screen,it's instant death,or the scrolling screen can push you off platforms if you're moving through the level too slowly.
The game can be challenging,but never too hard.
The enemies are in good variety,from regular Koopas(walking turtles)and imps(basic enemies that areslow moving creatures),and there's also flying Koopas,enemies who wear american football player uniforms and they can withstand multiple hits from jumping attacks or fireballs(but you can kill them instantly with the cape power up)but the football players can kick footballs at you(which can kill you)or come charging at you.There's birds that swoop at you from awkward angles,skeletons that keep putting themselves back together after you jump on them and they can also shoot bones at you and it can catch you off guard if you let them get close to you.There's giant hamsters that are difficult to avoid in narrow underground sections and they can't be killed by simply jumping on them,and my most hated of all enemies, that turle thing that flies in the cloud and constantly drops spikey projectiles at you that turn into spikey shelled turtles you can't jump on them and if they hit you,you will die or lose your power up.
You can use Luigi now too,and when he jumps,he'll float longer in the air which can be an advantage or disadvantage depending on the situation and your timing.
The levels are colorful and have a cute artstyle,you'll see cute forest areas and the cute looking hills and clouds with faces on them,mountains,caves(some of them are made from ice),castles,ghost houses,underwater,high in the sky etc.
The music sounds like the SNES version but clearer.All the memorable tunes are back and you'll hear the memorable music when Mario powers up from a mushroom,flower or cape.But,it's annoying when Mario says that ''momma mia''everytime he dies,I wish they didn't include that part.
Overall,this 2d platformer is one of the best 2d platformers I've played,it's my favorite Mario Game,and it's so fun and charming and the fact you can save your progress at any level makes the game almost perfect.